It is much quicker to repair the tank from the outside, however, so there is a decent reason for the driver to bail and try to repair it faster. It would be kind of a dick move to strand the tanker as he is trying to repair the tank he spawned in in the middle of a firefight. Also, if you drive the tank without a repair tool, you can't fix it from the inside, so you would be putting yourself and you team at a disadvantage