Our guitarist brings three guitars to each gig - a Les Paul (or similar, but usually a Les Paul), a Strat or Tele and something with a tremolo. Works for him and he has good tonal reasons for using all three.
I on the other hand use one bass, with a backup in it's case off-stage. I find I can get enough tonal variation by changing my hand position and playing style, and sometimes the volume controls on the bass that I don't need a second instrument, and I can go to drop D for the songs in Eb standard, negating the need for another bass.