@hawkeye10,
I think it could be the straw that broke the camel's back for some people. There are people who liked the "old" McCain and who just can't quite tell if Candidate McCain is just bullshitting to get to the White House and he'd be a perfectly good president, or if their fundamental understanding of McCain needs to be re-examined.
Those people tend to like Powell, too. If Powell rejects McCain and goes for Obama, I think that might help along some re-examination in that group. And either keep 'em home or maybe persuade them to vote for Obama.