My wife and I have ridden two up at the Bash for three trips now.
Its a double edged sword for me; 1) it slows me down and takes away some of my 'daring'. 2) it slows me down and takes away some of the likelyhood that I will wreck or get a ticket - she has a wonderful, built in speed control of slapping my helmet!
Riding a Busa, two up, on the Dragon, in the rain?
Priceless... and I have done it!
That said, if'n my wife had a bike I would MAKE her take it!