It *would* deter them from voting for Trump.
It would hurt both candidates.
A scrupulous person has qualms about promising they would support the winner even if it’s the likes of Jeb.
Of course it’s not enforceable but scruples are scruples.
Yes, but I think they’re trying to keep non-Republicans out. Of course, they may anticipate that Cruz and Trump voters, and other conservatives, might not want to sign it. Maybe that’s the strategy.