You're right, it definitely sends a message, but at what cost? Is sending a message worth electing Hillary? Are republicans willing to pay that price and then bitch about how bad she is afterwards?
Example, I may not agree with Rudy on everything, in fact, Antoninus brought up several points on that topic, but I would definitely get out and vote for him if he's the nominee. And "republicans" that don't, aren't republicans. You vote with your feet...and a no vote is a vote for democrats...simple as that.