I've seen a few people suggest that it'll be a disaster for the Dems if the primary drags out until the convention in August. See, for instance, Ed Morrissey, who predicts a "coming meltdown" if, say, superdelegates hand Clinton the nomination, black voters revolt, McCain gets ample time to win over conservatives, and the Dem nominee has only a short window to raise funds and consolidate support. That's an extreme scenario, but I've heard other versions, too.
I'm not sure. One could, alternatively, imagine that the absence of a clear Democratic opponent would make it much harder for McCain to start attacking (back in 2004, the GOP was able to coalesce around the Kerry flip-flopping meme early on, which gave it time to sink in). Meanwhile, it seems that as long as the Democratic nomination is up in the air, dissatisfied conservatives are more likely to spend time airing their grievances with McCain than training their fire on his opponent.
Clinton and Obama would also have time to sharpen their message on the economy, in a debate waged largely on their terms—McCain's ability to change the subject here would be minimal. And they'll presumably get positive press so long as it's a horse race. Obviously there are wild cards (if the Clinton-Obama slugfest got much, much uglier, that would hurt; and Morrissey's right that it would be a fiasco-and-a-half if superdelegates unfairly swung the race, or if the Michigan/Florida controversy explodes), but on its own, a drawn-out Dem race isn't necessarily a problem for the party—and might even be a boon.