The only bass-free bands that I can think of that wouldn't make a huge sonic improvement by adding a bass player are those featuring an organist or other type of keyboardist who is VERY good with pedal bass lines or left-hand bass lines.
Some of those have already been mentioned my list would include Soulive and the Robert Walter Trio, or any of the classic jazz organ trios with Jimmy Smith, Jack McDuff, etc.
And another exception - Charlie Hunter, who plays guitar and bass line simultaneously on the same instrument. But he's an exception/freak of nature. And his stuff is great.
Some of the guitar/drum duos are kinda' cool, particularly when I first hear them - I appreciate the rawness and DIY attitude. But their lack of sonic versatility just gets monotonous after a while IMO. The lack of a "bottom" then becomes very apparent - annoying present for its absence.
