I see music theory as a way to convey musical ideas to others who can't get inside my head and hear what I'm hearing. Over hundreds of years, people have labeled everything about music and they've taught others that when they hear "Authentic Cadence", they think 5-1. If I'm playing with a group of people, and we're just jamming, I don't want to have to say, "Alright guys, we're gonna make every note a little higher here...umm, it's about 7 frets higher, just listen and figure it out." Why say that if I can just say, "Key change : Dominant." and people get it make the proper adjustments? As Chris said, who cares if my A is your A-flat, (As it is when you play with non-concert pitch instruments) as long as everyone knows how to mix together right.