New HRC campaign rationale -- Obama might get shot and killed before formally securing the nomination, so she may as well stay in the race!

UPDATE: On a more serious note, the difference between the current race and other previous campaigns that may have lingered on into June is that given this year's primary schedule there simply aren't enough delegates left at stake for future primaries to make a difference. If she were holding out for a June primary in California that she thought would let her catch up, that'd be a very different story from the actual "waiting for Puerto Rico" scenario we're currently in.