Mike,
Actually,I have a 2 year degree in music, so theory isn't all that new to me. So I know about enharmonic equivalency and such. I still struggle with things like chords subs and other things, but overall I have a pretty good grasp. I have to not necessarily disagree but rather differ with you in what I was taught about intervals. I was taught that for things like 9ths and 11ths, you take the degree of the scale, ie the 2nd, and add seven = a 9th. Hence the root note being an octave etc. Obviously you don't have to voice them an octave higher, but in some situations like the chord that Truman used as an example, having a G next to an F##(G#) is a tight and very dissonant interval of a minor 2nd. So by playing one of them an octave higher, it gives the interval a bit of breathing room so to speak. Some things work better on the piano than guitar because of the ability to spread out the voicing. BTW, I love to use sus chords. Something I picked up from being a Pete Townshend fan. This is great. I love to talk theory because it makes you think about how you're playing and how the notes you use work together. Whoever started this board deserves a pat on the back.
davey O.