#NeverTrump
The threat of the Never Trump voters - one out of every five Republicans - is a reminder that political coalitions tend to be zero sum: reach for a new group of voters and you risk losing a group of older ones.
Clinton or Trump will be President. Actively working to sabotage Trump if your candidate loses the nomination is the moral and intellectual equivalence of voting for Hillary.
You can rationalize it all you want, that does not change the basic facts.