6-string steel guitarists use open tunings all the time. I think one of the most popular open tunings is DADGAD; I see references about it a lot in the music newsgroups and forums.
Guitar pieces using open tunings are pretty easy to identify - they seem to have a new-age sameness to them, never change keys, and don't stray too far from the root. This is not universally true, of course. For example, Pierre Bensusan is an astounding guitarist who just happens to compose and play in DADGAD.
|Musical Notes c.1980 Art Sulger|
I've found that the natural tuning of the guitar, EADGBE, is very adaptable to most of the 12 major, 12 minor, and various modal keys, as well as serial music (I'm guessing about the Serial Music part).
One of the problems with open tuning, if you are a musician that reads music, is the translation of score to sound. It's hard enough to read music, without having to translate it into one or another open tunings. Also, for a 12-stringer, I expect that performing in various open tunings would take a lot of setup time between songs, and might require that you travel with a guitar in each tuning.
If you are a composer of guitar music who often works things out on the instrument (like I do, unfortunately), you are giving up a lot of tonal landscape by not using standard guitar tuning.
I try to avoid open tuning for the same reason I avoid marijuana: it makes everthing you play sound good. You can probably get a few good tunes out of each open tuning, but unless you have Bensusan's or Kottke's talent, your tunes will soon sound all alike.
Oh, and I heard the Leo Kottke now avoids open tuning.