Having Sarah pinch-hit would be a big mistake. It would make McCain look like he’s afraid to debate Obama, or maybe even be seen as a sign of disrespect.
If I were McCain, I would show up, do the debate, and put Sarah in the audience, front row center. This would distract The Obama so much he wouldn’t be able to get anything out except “um, uh... uh, breathlizer... uh... um.....”
I agree. Besides, it just isn’t done, this is a Presidential debate, the people want the two PRESIDENTIAL candidates debating. Sending Sarah is not an option in a Presidential debate.
If Obama and the LSM had been claiming that "well, Palin is good, but she's not at the top of the ticket" then you'd have a point. Since Obama et al. have claimed that she's an unqualified lightweight, then he should be able to clean her clock easily. Unless, of course, he's an even-less-qualified lightweight.
That having been said, I would suggest that the right approach would be to announce that if there is a debate, Palin will be attending, and Obama/Biden can decide which of them, if either, is going to debate her.