Considering the math of the delegate count, why is Hillary Clinton staying in the race?

Statistically it is almost an impossibility for Hillary to come back and reclaim the pledged delegate lead. So why is she still in the race? Is she only hoping to bloody him up to the point where he cant beat McCain? Then her and Bill can parade around yammering about how they told us he was unelectable. Setting up another run for her in 2012?