The Atlantic's generally excellent Marc Ambinder asks a question whose answer, I think, is pretty obvious:
The [Democratic] superdelegates want a "winner," a candidate who can beat John McCain winner and not a "who got the most delegates or votes" winner. If HRC wins Pennsylvania by a healthy margin and stays in, by the time June 3 rolls around, she's likely to have closed the popular vote gap a lot and the delegate gap a little.
But the superdelegates won't have any additional information about who is best positioned to take on John McCain.
Rationally, why would they decide in June? Wouldn't it make more sense for them to wait as long as possible to see how each candidate polls against McCain or fares in the press over the summer months?
Well, it would, except that the longer the primary goes on, the more poorly both Democratic candidates poll against John McCain. If what you're looking for in November is the slightly-less-damaged of two candidates who have been driving each others' negatives up for months while an unchallenged McCain basks in the media's warm glow, then Ambinder's suggestion makes perfect sense. But I don't think that's what most Democrats are looking for.