And remember, he will have to disembark along with the rest of his unit (unless it is a Big Mek, which has the Independent Character rule). Also, you can't disembark and embark on a different vehicle in the same turn, so they will have to stand behind the wagons for a whole round of shooting just for an extra 33% chance to change a tire.
But don't let me talk you out of it, I love completely undoing a whole turn's worth of opponents' anti-tank shooting, even if it isn't strictly the best tactical decision.