In 1998 I suggested that the MB, having been written by the Chafeitz Chaim, reflects an attitude where the line between halakhah and personal improvement is intentionally blurry. To a ba’al mussar (although the CC was not an adherent of the movement), halakhah can be viewed as the bare minimum of a mussar regimen. Mitzvos exist to hone oneself, but someone who is serious about this task would try to harness them consciously toward that end, would commit to other practices toward that goal, etc… So, we can view “ba’al nefesh yachmir” as mussar advice, but that doesn’t stand entirely separate from pesaq.
Although… it’s not 100% clear that the Mishnah Berurah was even written with the intent that it be used as a practical halachic guide. The Chafeitz Chaim writes in his introduction (traditionally found in the beginning of volume IV, tr. Rabi Seth Mandel, posted here):
First is that the SA by itself without learning the Tur is not comprehensible, because when he wrote the SA it was the BY’s intention that people should first learn the sources of the halokho in the Tur and the BY, so that he would understand the reasoning and logic of each shitta and the practical differences between each… Many times it happens that the SA combines in one s’if something that is only l’khat’hilla with another that is b’di’eved and l’iqquva, something that is d’orayso with something d’rabbonon, and there will be a difference if there is a safeq etc… But learning every din in the SA with its sources and reasons from the Tur and the SA is too great a task for most people nowadays… since in this way one medium siman may take several days and sometimes a few weeks…
The second reason… is that it is difficult to know the halokho l’ma’aseh because of the multiple disagreements brought by the acharonim… and even if he would want always to be mahmir in the matter, that is also not a safe way, because sometimes it will be a chumra that leads to a kula. I also see that from the time the B’er Heitev summarized the Taz and the Mogen Avrohom and others and responsa about 150 years have passed, and in the meantime there have been very many famous g’onim who have dealt with the matters, such as the Elya Rabba, the Matteh Y’hudah, and many others, and the Sha’arei T’shuva only brings a little bit of this in some places. In particular, the Pri M’godim, which is a great work and deals in each siman with new questions l’ma’aseh, and whose conclusions have been accepted is almost not quoted almost at all in the Sha’arei T’shuva… and similarly many many other famous g’onim whose views have been accepted after the Sha’arei T’shuva was printed, such as R. ‘Aqiva Eiger, Derekh haHayyim… So that now if a person wants to understand some halokho l’ma’aseh that is not fully discussed in the SA, he will have so search in many acharonim… Therefore I have strengthened myself with the grace of G-d to fix these matters. I have written an explanation to the SA that is sufficient in my opinion… and explained each din in the SA with its reasons and logic from the g’moro and posqim… and in each matter where there are disagreements among the posqim I have presented the conclusions of the acharonim (gathered from the BaH, the D’risha, the Elya Rabba, the G’Ro the P’ri M’godim…)
It appears that the purpose of the book was not to provide his own ruling, but to survey the later posqim who have added complexity to the field so that someone looking to reach a decision knows who wrote on the matter.
Yes, the CC often gives his own opinion, including our “ba’al nefesh yachmir“, but it is unclear to me he intended that opinion to be a pragmatic ruling rather than a theoretical statement. This would explain why the Mishnah Berurah’s rulings diverge from accepted practice so much more often than the Arukh haShulchan (a contemporary work from the same region). Halakhah lemaaseh, pragmatic rulings, need to take such precedent and continuity into account; discussions of textual theory do not.
Dr Haym Solovetichik, in his famous paper “Rupture and Reconstruction“, describes the difference between the two as follows:
This dual tradition of the intellectual and the mimetic, law as taught and law as practiced, which stretched back for centuries, begins to break down in the twilight years of the author of the Arukh ha-Shulhan, in the closing decades of the nineteenth century. The change is strikingly attested to in the famous code of the next generation, the Mishnah Berurah.6 This influential work reflects no such reflexive justification of established religious practice, which is not to say that it condemns received practice. Its author, the Hafetz Hayyim, was hardly a revolutionary. His instincts were conservative and strongly inclined him toward some post facto justification. The difference between his posture and that of his predecessor, the author of the Arukh ha-Shulhan, is that he surveys the entire literature and then shows that the practice is plausibly justifiable in terms of that literature. His interpretations, while not necessarily persuasive, always stay within the bounds of the reasonable. And the legal coordinates upon which the Mishnah Berurah plots the issue are the written literature and the written literature alone.7 With sufficient erudition and inclination, received practice can almost invariably be charted on these axes, but it is no longer inherently valid. It can stand on its own no more.
6 Israel Meir ha-Kohen, Mishnah Berurah. This six volume work, which has been photo-offset innumerable times, was initially published over the span of eleven years, 1896-1907, and appears contemporaneous with the Arukh ha-Shulhan. Bibliographically, this is correct; culturally, nothing could be farther from the truth. Though born only nine years apart, their temperaments and life experiences were such that they belong to different ages. The Arukh ha-Shulhan stands firmly in a traditional society, un-assaulted and undisturbed by secular movements, in which rabbinic Judaism still “moved easy in harness,” R. Israel Meir Ha-Kohen, better known as the Hafetz Hayyim, stood, throughout his long life (1838- 1933), in the forefront of the battle against Enlightenment and the growing forces of Socialism and Zionism in Eastern Europe. His response to the growing impact of modernity was not only general and attitudinal, as noted here and below, n. 20 sec. c, but also specific and substantive. When asked to rule on the permissibility of Torah instruction for women, he replied that, in the past, the traditional home had provided women with the requisite religious background; now, however, the home had lost its capacity for effective transmission, and text instruction was not only permissible, but necessary. What is remarkable is not that he perceived the erosion of the mimetic society, most observers by that time (1917-1918) did, but rather that he sensed at this early a date, the necessity of a textual substitute. (Likkutei Halakhot, Sotah 2 la [Pieterkow, 1918].) The remarks of the Hafetz Hayyim should be contrasted with the traditional stand both taken and described by the Arukh ha-Shulhan, Yoreh De’ah 246:19. One might take this as further evidence of the difference between these two halakhists set forth in the text and documented in n. 7. One should note, however, that this passage was written at a much later date than the Mishnah Berurah, at the close of World War I, when traditional Jewish society was clearly undergoing massive shock. (For simplicity’s sake, I described the Mishnah Berurah in the text as a “code,” as, in effect, it is. Strictly speaking, it is, of course, is a commentary to a code.)
7 Contrast the differing treatments of the Arukh ha-Shulhan and the Mishnah Berurah at Orah Hayyim 345:7, 539:15 (in the Arukh ha-Shulhan) 539:5 (in the Mishnah Berurah), 668:1, 560:1, 321:9 (Arukh ha-Shulhan) 321:12 (Mishnah Berurah). See also the revelatory remarks of the Arukh ha-Shulhan at 552:11. For an example of differing arguments, even when in basic agreement as to the final position, compare 202:15 (Arukh ha-Shulhan) with 272:6 (Mishnah Berurah). This generalization, like all others, will serve only to distort if pushed too far. The Mishnah Berurah, on occasion, attempts to justify common practice rather unpersuasively, as in the instance of eating fish on Sabbath, (319:4), cited above n. 3, and, de facto, ratifies the contemporary eruv (345:7). Nor did the Arukh ha-Shulhan defend every common practice; see, for example, Orah Hayyim 551:23. (S. Z. Leiman has pointed out to me the distinction between the Arukh ha-Shulhan and the Mishnah Berurah is well mirrored in their respective positions as to the need for requisite shiurim in the standard tallit katan, noted by Rabbi E. Y. Waldenburg in the recently published twentieth volume of his Tzitz Eliezer [Jerusalem, 1994], no. 8, a responsum that itself epitomizes the tension between the mimetic culture and the emerging textual one.)
I am suggesting, though that this shift deemphasizing mimetic tradition (the flow of practice transmitted culturally) in favor of a near-exclusive focus on texts was not actually part of the Chafetz Chaim’s worldview as much as those post-war rashei yeshiva (such as R’ Aharon Kotler) who promoted the idea of using the Mishnah Berurah as a poseiq acharon. The Chafetz Chaim wrote textually because he was intentionally giving a survey of textual theory.
As further evidence that the Mishnah Berurah was not intended to be a practical law guide, we have a lot of testimony that shows that its own author often followed the common Lithuanian practice over his own “ruling”. Despite the origin of wearing one’s tzitzis strings out being in the MB, the CC did not. His qiddush cup doesn’t hold as much wine as the MB would require. (It is still in the hands of the Zaks family and has been checked repeatedly.) He used city eiruvin for carrying on Shabbos. The Chafeitz Chaim did not say “Berikh Shemeih” when taking out the Torah. Etc…