There are 4 possible situation. 1 - McCain shows up, debate
2 - McCain shows up, no debate
3 - McCain stays in Wash, no debate
4 - McCain stays in Wash, debate
I predict #4, giving the McCains' efforts to turn this entire process into a 3- ring circus, with McCain playing 'zee maverick' to Palin's bearded lady.
As for who benefits, McCain's now a loser whether he shows up or not.