@solargarlic78 said:

I totally get the dissonance of thinking of the I as the V (because the V wants to resolve etc etc.) But insofar as the Grateful Dead inaugurated jams music with Dark Star (A Mixo), I like to think of that as jamming on the V chord in D major scale. (This all has to do probably with how I learned my scales on guitar and then tried to transpose them to modes). So much jam music is this jamming on the V chord imo.

I think your last point is the main difference in the way you see this versus how Mike and I do, and totally makes sense from a practical standpoint. Your viewpoint is entirely consistent with the way you approach mode, that is, you "learned your scales on guitar and then tried to transpose them to modes." Jamming on the V chord in D major is, of course, the same pitch collection as an A mixolydian mode: D, E, F#, G, A, B, C#, but oriented to begin and end with A. So you're looking at the fret board and thinking "OK, I play a D major scale but jam around the fifth scale degree." You have D major in mind because you are thinking of pitch collections, not harmonic functions.

It also has to do with your terminology, which again is totally consistent with the way an improvising rock guitarist thinks about notes versus the way a classically-trained theorist does. What you call "jamming on the V chord" I would describe as "jamming on scale degree 5 in a D major scale." The difference is in the subtle significations of "V" versus "5." One connotes a scale degree and essentially melodic function, the other is a harmonic function (dominant), and essentially harmonic function. There is no phantom D major chord holding everything together in "Dark Star," it's "in A" as much as "Disease" is "in A." The fact that you jam on A mixolydian, and think of it as jamming on scale degree 5 in a D major pitch collection, does not take away from the "A-ness" of Disease's key.

I hope this makes sense and also doesn't come off as pretentious or derogatory! I'm a teacher, specifically a music history teacher, so it's difficult for me to talk about this stuff sometimes without sounding pedantic. But my point is that we all are hearing this the same way, but we're just using different terminology to describe it, and that terminology is most determined by what our introduction to music theory was. If your introduction to music theory is a typical classical music college course, then you're going to describe it differently than if your introduction to music theory is trying to understand how to work your way around a guitar to play improvisational rock music.