Adriaan, what I intended to mean was that many horn players arrange their songs in Eb or Bb (check Coltrane, for example, how many tunes he did in Eb? A lot, if remember right...). So if you had to play in a Band that plays most of their songs in Eb, it would be kind of natural tuning down a half-step, although I'd also prefer to transpose myself (I'm not a guitarrist and didn't like to use open strings on my riffs).
To be honest, I don't know much about Stevie Ray and was just thinking out loud about the reasons some guitarrists would opt using low tunnings. In fact, there is a lot of Metal guys who play lower than half-step, and I'm pretty sure it doesn't have anything related to playing with Horns. LOL.
You are right when say it relates more to the strech than to transpositions issues.