If chord voicings don't matter, why have two different chord symbols for the pitch collection A C E G? It could be Amin7, or C6, depending on context. Or what about A C Eb G? A-7(b5) or C-6. Or what about D/C? It could "mean" Cmaj7, or it could mean D7 in third inversion.
Remember, chord symbols weren't invented as devices for theoretical analysis - they were invented in the 1920's as an easy means to convey basic knowledge of harmony for the purposes of improvisation. They weren't supposed to be an all-purpose means of analyzing and codefying harmonic practice - they were just a shortcut for jazz improvisers. There are different practices associated with different chord symbols depending on the stylistic norms.
I've had this argument with a lot of people. They emphatically argue that there shouldn't be any difference between Cadd9 and Cadd2, because within jazz specifically there usually isn't any distinction between the 2 and the 9 within harmonic voicings. The problem with this is quite simply that there are standard practices that don't conform to jazz paradigm. There are other forms of music besides jazz (when was the last time you saw Cadd9 on a jazz chart, anyway?) and they have their own ways of interpreting chord symbols. Country and country rock players have their own way of understanding these two chord symbols for the reasons I outlined previously, and no amount of guffawing and harumphing from the jazzers is going to change that basic fact. You can't argue with the truth except for blithely saying "thats not the way it SHOULD be!" According to whom? Jamey Aebersold? Please.
REMEMBER. Chord symbols are NOT theoretical tools. Most of the time they are painfully lacking in analytical ability, and serve to convey basic information about the harmony. Some chord symbols may convey more information than others (voicings, extensions, inversions, etc) but they aren't the be-all-end-all-truth on harmony.
One thing that can be very easily gleaned from your post is that you, and most people, put way too much faith into the power of chord symbols to explain what is going on harmonically. You give all sorts of possible exceptions to the Cadd2, Cadd9 idea, but in doing that you open up a whole wide range of other questions.
Consider a big band voicing, if you will. The trombones are playing a G major triad in first inversion, the trumpets are playing a B-7 third inversion (F# on top), and the saxophones are playing a spread C major 6 with the first alto playing an A on the staff. The ear very clearly hears the composite of this event as a "C major 7" sort of sound, now doubt about it, but in order to understand what's truly going on, you have to analyze the voicing. Sure you could just go ahead and say that its just a "C major 7 (9,#11,13)", but there's way more than that chord symbol can give you happening within the chord. It's not just "color", its the very intervallic nature of the harmony.
The Cadd2/Cadd9 idea attempts to control that, and give some idea of the voicing the composer wants. If the composer/arranger wants that lapsteel effect, how else are they supposed to notate it? Are they just SOL, and are subject to the whims of the overzealous jazz improviser, overembellishing the chord to their own desire?
I think you completely understate the nature of voicings within composition and music, because to a jazz improvisers, voicings are interchangable. That simply isn't the case with a lot of other forms of music.
I think you misunderstand. I never said that voicings are interchangeable. If they were interchangeable, there would be no point in having more than one. They sound different, but more often than not, they don't change the chord name when you are talking about a different arrangement of the exact same notes. (There are of course a times when they do, as in the m7/M6 example you gave, but IME add2/9 isn't one of them.) That is, they are not interchangeable in terms of how they sound, but they are often interchangeable in terms of chord nomenclature.
Yes, of course you can build a big complex-sounding chord out by having the horns and woodwinds play different chords, but so what? Those are different sets of notes. What does that have to do with interpreting the same set of notes differently, which is precisely the question with add2 vs add9? What you describe is an important point, but it's irrelevant to this particular question.
You're arguing that chord symbols of the type we're addressing here are not rigorous descriptors of what's going on harmonically, but then you treat them as if they are rigorous descriptors of voicing. They are no such thing, in any practice I know of (and no, I'm not just a jazz guy--where do you get the assumption that the only people who could disagree with you on this are "jazz guys" anyway?). If anything, chord symbols are FAR LESS rigorous at describing voicing than at reflecting harmonic detail. If you see "C" you at least know there's C E G involved, but you are told NOTHING about voicing. It seems a pointless quibble to claim that all kinds of, say, major 7th voicings can be described as the same chord, yet the most minor of voicing differences in an add2/9 chord, which can't even be conceptually maintained in any rigorous way once you get away from a single instrument (and often not even with a single instrument), must be distinguished by two different chord names. If chord names are an inexact tool, as you say, why kill yourself trying to use them as if they were more exact than they are, especially when you don't extend the same rigor to other chord names?
You misunderstand what I'm saying. I'm not saying it SHOULD be this way. Maybe it would be great if you could specify every chord voicing with a different chord name. (But where would you draw the line?) But, because of the very inexactitude you point to, you can't do this. I'm saying, whether it should be this way or not, it mostly IS this way IME and AFAICS. You seem to be positing a widespread, if not universal, agreement about the distinction between add2 and add9. AFAICS, that agreement doesn't exist. You might wish it did, you might think it should, but I see little reason to think it does, even in country or rock (both of which I have some experience with, BTW). You seem to be suggesting that there are different chord symbols, or a fundamentally different understanding of those symbols, in country or rock as opposed to jazz. IME this simply isn't true. We're not talking Nashville numbering here, we're talking the language of C, C7, Gm9, etc. Those don't vary AFAIK, though stylistic choices do (as when a jazz guy sees a C on a chart and elects to play, I dunno, a C69 instead--he doesn't think that C actually symbolizes C E G A D, he still knows C = C E G; he just knows he can sometimes make that substitution if he feels the context warrants it). A C is still C E G, not necessarily in that order.
FWIW, I've been a pretty serious guitar player, as well as a bassist, for quite a while. I certainly can't claim to have seen all the charts there are, or even some major fraction, but I've seen my share. And I can tell you that by and large, guitarists don't expect to get detailed voicing information from chord symbols. That's generally not the sort of information they expect chord symbols to convey. It's a job for which chord symbols are very blunt tools indeed--blunt to the point of being almost entirely ineffective. Suppose you want the guitar player to play a C in a very wide voicing. How do you do that with the chord symbol "C"? You can't, and no guitarist with a grain of sense would expect you to be able to. If a composer wants a certain voicing, he or she has the option of writing it out in notes--just as you would for horns or woodwinds--or adding some verbal description ("play reeeaalllly high up on the neck"). Just about the most you can get out of chord symbols in terms of voicing is with slash chords. But even then, they generally give you only one piece of information about the chord and don't specify the intervallic relationships of all the other notes in the chord. Thus, for a Dm/C#, you do know at least that the C# is on the bottom, but other than that, you don't know anything at all about where the other notes are in relation to each other or the bass note.
IOW, basically, with few and incomplete exceptions, chord symbols don't "do" voicing. They weren't designed to. So it's hard to see why one should bother to pretend that they do. The very inexactitude you point out, stemming from the genesis of chord symbols as shortcuts, is precisely why they don't do voicing.
You never did address my specific examples and questions about Cadd9 versus Cadd2, by the way.