Well when we do it they usually are fodder shields to block fire for the weaker vehicles that want to get in range. Infnatry that bails out of destroyed vehicles seek refuge in the passing trucks until they reach a building entrance, they offload the infantry inside and block the entrances to help. Another good tactic is hampering the enemy vehicle bay so that they can't field thier tanks properly, they just bump into the OTs and get delayed that way. Plus they can't buy more than one vehicle otherwise the blocked one gets destroyed. So even as is, the Ore Trucks are definitely a tool to be used, a funny one, but a lot of times their main goal is to just delay the enemy so that reinforcements can come in for a second attack.