If Clinton does win the nomination, somehow, and indictments come down from the FBI for her mishandling of classified information before the Convention in July, it is IMPERATIVE that Bernie hold as many earned delegates as possible for any convention floor fight that might ensue. The more delegates, and actual votes Bernie holds, the better.
As @dsully has pointed out in his well research diary Laws Hillary Might Have Violated, there are plenty of reasons to believe that damocles sword could fall soon and cause the other shoe to drop so hard. Bernie hasn’t been willing to say anything because he’s willing to let the FBI investigation run it’s course, but I suspect he knows more than he’s letting on and insiders are telling him to hang in there till the end.