At least he was honest. I guarantee you there were candidates up there who won't support the eventual nominee (and most of them wouldn't support Trump), but none of them raised their hands.
Change the question ever so slightly, ask "How many of would support Trump as the GOP nominee against the Democratic nominee?" Why put Trump directly on the spot, unless the point is to "get Trump"?
Obviously, honesty is no way to keep the RNC in line. The efforts to marginalize Trump haven't even started in earnest, yet.