That doesn't matter because one of them will become president. It's like if I put two foods in front of you, neither of which you like... You will get hungry enough and eventually you're going to eat the one that you dislike the least.
If Hillary gets the nomination (and she will) and every Bernie supporter chooses to be stubborn and not vote, they're leaving it up to everyone else to decide a fate that will affect them also. Voting isn't about exclusively voting for someone who you agree with on all policies, it's assessing the candidates and picking the one that you think is the best fit out of the field.

It's like people who call themselves Agnostic when asked about religion. Just being "Agnostic" isn't a thing. It's not even logical to have no belief at all (unless you're mentally incompetent). You're either an Agnostic theist or an Agnostic atheist. Everyone has a belief or a preference to things, it's just a matter if they'll accept it, act on it, admit it, etc. So yes, I think if you refuse to vote on the grounds that your preferred candidate didn't get the nomination, you're essentially refusing to accept that one of them is the better choice.