Who'ever gets in the tank first should be able to lock it IMO. Nothing more frustating than getting someone aboard with his mind set on ruining it for you.
Team tankers will still be able to do their thing, cause they can simply choose who they want onboard.
Ensures good & interesting gameplay if you ask me.
I am afraid that tank class should be like sniper, no one have big chance to get it. You are not friend? Than get away, you are not welcome here, so this is how it end?