The reason?? A guitar in standard tuning is a fairly feeble instrument, in my opinion.. I like open tunings much more.. They make you sound better than you really are!! LOL!!

Accomplished? Well, depends on how open you want your open D to be... I tend towards DADGAD tuning myself... but some folks get by with DADGBD Or DADGBE even...Open D is often DADF#AD

Any tune?? well... ya provided you want to play it in the Key of D... Actually DADGAG played in the key of G sounds really cool, with some lovely unresolved chords and such...

Mulit-instruments? Yup... saves on wear and tear on strings, and it gives one ax a chance to cool off while you play the other one! {~`

Capo?? Capo away!! Won't hurt a bit...

Pitfalls?? None that I can see... The broader your horizons the better.. just try not to become too reliant on one specific tuning... Try not to get to the point where you have to say, "sorry, can't play that song... I don't have my open Bb guitar with me tonight.... "

Other stuff... Ya... check this out for a few more tunings to try... http://www.guitarists.net/tunings.cfm

Enjoy eh!

{~`