Some may wish to invoke the principle of חזי מאי עמא דבר (see what people do) in the case of daily ברכת כהנים, namely that it is wrong to attempt to reinstate daily ברכת כהנים where this was not previously practiced, because there is a rule that we have "see what people do" and follow the מנהג as such.
In this article we will search the halachic literature for some examples of where this principle is applied or not.
1
There is a question in the [1] 'גמ what ברכה ראשונה to make where מזונות, העץ, or האדמה are not made. The תנא קמא says שהכל.
רבי טרפון says בורא נפשות רבות וחסרונן.
רבא בר רב חנן asked either אביי or רב יוסף what the הלכה is; he was answered to see what people do.
2
Normally, one does not answer אמן after one's own ברכה. There is a צד in the 'גמ, however, that אמן should be said after certain ברכות [2].
תוספות observes, as a conclusion, that if we look at what people do, we will see that they only say אמן after בונה ירושלים in ברכת המזון.
3
Some had a מנהג to write various פסוקים in their מזוזות, in addition to the obligatory פרשיות.
מעשה רקח [3] says that the consensus of all the פוסקים is not to add anything, and indeed this is supported by the fact that this is what (most) people do.
4
The רשבא writes in a תשובה [4] that in some communities there is a מנהג that on ליל ראש חודש between חצי קדיש and שמונה עשרה of מעריב, the שמש announces "ראש חודש". This is not a הפסק because it is a צורך התפלה, so that people should remember it is ראש חודש and say יעלה ויבא. This is what people do, and no one has ever protested.
5
All the פוסקים mention that המפיל should be said with שם ומלכות. However, some סדורים print it without שם ומלכות, although none of the פוסקים make explicit mention of such a possibility. This being the case, we revert to the principle of seeing what people do only where the הלכה is רופפת בידך; where the הלכה is clear-cut, however, the fact that we have not seen this הלכה kept is not a proof of its non-applicable status. [5]
6
The רשבא has another תשובה [6] in which he discusses whether or not to make קידוש by the 3rd סעודה on שבת. He barely even weighs the pros and cons of the issue before saying "look what people do - no one makes קידוש at the 3rd סעודה".
7
In general, רב פעלים [7] points out that in order for what people do to have any weight, their מנהג must be known and certain.
8
אדמת קודש [8] presents a שאלה as to where to make a ברכה on putting up a מזוזה. After reaching a conclusion, he says that "this is all להלכה ולא למעשה, and in practice, if the הלכה is unclear, one should see what people do".
9
כף החיים [9] presents a discussion as to whether or not one may shecht birds sitting. He concludes to see what people do.
10
The רמא [10] has a dilemma as to whether one שמש is sufficient for an entire household to not be considered benefitting from the light of נר חנוכה, or if each individual מנורה requires its own independent שמש. He concludes that his observation of common practice is for each individual מנורה to have its own שמש, after which he then supports his conclusion with halachic reasoning.
Conclusion
חזי מאי עמא דבר is a halachic principle that, like all halachic principles, applies in some places and not in others. It is up to the the discretion of פוסקים where to apply it or where not.
In general, it seems that פוסקים apply this principle in cases where the הלכה is not clearly established, at which point we observe the practice of the general population, by which we can assume that the common practice is based on a פסק that was accepted widely. [11]
There is a clear-cut הלכה that ברכת כהנים must be done daily, no ifs, ands or buts [12]. This הלכה is definite, certain, established, and undisputed. There are a variety of attempted justifications why not to but none of them hold much halachic water (see here). This is a far cry from רופפת בידך. Even the פוסקים who offer reasons why not to such that, according to their reasoning, ברכת כהנים is nearly an איסור (see, for example, here), hold that, in theory, we are מחוייב only that there are factors mitigating our התחייבות in practice. No one (not even the בית אפרים, who uses every possible justification to prop up the מנהג) apply this principle of חזי מאי עמא דבר to ברכת כהנים; so why should we?
Appendix
Logic would dictate a few caveats:
1) The people that we observe should be generally religiously learned and knowledgeable, by which we can assume that their halachic behavior has halachic import, and is not simply based on error or ignorance.
2) This entire rule only applies where the הלכה is not clear-cut and established. If it is - even where said הלכה is not commonly followed - then too bad on what people do; they must be taught to improve their halachic behavior.
3) If the הלכה is not initially established, at which point a פוסק may decide to follow the מנהג, said פוסק (or any other פוסק) may then decide to clarify the הלכה based on objective criteria and straightforward halachic reasoning, at which point the הלכה would become clear, and relying on the מנהג may no longer be permitted or necessary.
[1] ברכות מד.
שיטה מקובצת and ריטבא there point out that this is indeed how we pasken, in that we say שהכל before and בורא נפשות after. See ב"י או"ח רד:יא
ערבי נחל (בראשית ב:כח), however, has a deeper understanding of this 'גמ. He asks a very obvious question: why didn't רבא or רב יוסף say the הלכה explicitly? Why does he pin the matter on the מנהג? Can there never be such a thing as a מנהג טעות? (Ed. - This is a rhetorical question: there obviously can be; people are not perfect and, as such, are capable of making mistakes. For a fuller treatment of מנהג טעות, see here.)
This can be explained as follows:
All the creations were created by mechanism of a specific דיבור, such as "יהי אור". Hence ברכות הנהנין tend to focus on creation, for example, בורא פרי העץ or בורא פרי האדמה. On the other hand, water was created automatically through the very act of 'ה speaking, so the ת"ק suggests שהכל נהיה בדברו, which implies כביכול passive, as opposed to active, creation, in that speech is made (partly) through water (vapor).
Whereas רבי טרפון argues that water also was created specifically, hence בורא נפשות רבות; so where רבה בר רב חנן wanted to find out the הלכה, he was told "you can understand this point on your own by seeing מאי עמא דבר, meaning, what a person's speech consists of, namely, fire (warmth), wind (air), and water (vapor). So too הקב"ה's דיבור also consists, so to say, of fire, wind and water, which is why the הלכה follows the תנא קמא."
[2] ברכות מה:ב
[3] הל' תפילין ומזוזה וספר תורה ה:ד
[4] סימן תצג. I have seen many congregations where the גבאי announces יעלה ויבא in that exact spot.
[5] כף החיים או"ח רלט:ז:א
[6] חלק ז שנ:א
[7] חלק ד יו"ד לו:ו
[8] יו"ד יח:יב
[9] יו"ד כד:א
[10] שו"ת פא:ב
[11] פרשת דרכים טו:כד; see also אמרי בינה חלק רביעי חקרי לב ה
[12] see, for example, רמבם in the opening lines of הלכות תפלה וברכת כהנים, ספר החינוך - מצות ברכת כהנים בכל יום, and משנה תענית ד:א which presumes, as obvious, that ברכת כהנים is done daily, and simply addresses at which תפלות this דין applies.
In general, many of the דינים of ברכת כהנים are based on the presumption of its daily occurrence.
