I knew McCain would show. That way he can spin it, "McCain, out of his rigorous resolve to solve this crisis, has actually taken time from this pressing issue to go debate. What a hard worker!"
Plus it covers his ass if people see him leaving the debate as a copout.
Even better, if he fucks up at the debate, it can be spun that McCain was so busy working to solve the issue that his mind wasn't on the debate, which is a waste of time but he had no choice because that talker Obama wanted a debate so badly. See, McCain is capable of cooperating with the "other" party.