For any dumb Republican who would do this I think some would vote for Obama, thinking an inexperienced black man can't win, and others would vote for Clinton thinking of all those things Sean Hannity has drilled into their heads over the years.
At the end of the day it is meaningless.
Kos tried to get Democrats to cross over and vote in Michigan. Exit polling that I saw suggested that Democratic voters made up less than 1/2 % of those who voted in the Republican Primary.
I think most people just stayed home, as what I have read so far indicated that GOP turnout is low.