it should mark everything but for lock-on the player with the designators should have to keep it on the target for the rockets to hit
Previous games did it like this and it people didn't use the designator because of it. If you need to keep holding the lock, you need to be vulnerable for an extended period of time. This is pretty much asking to be shot, as the red laser alerts anyone that's even barely looking at you. In addition to that, when the vehicle ducks behind cover, the lock is broken and the engineer doesn't get the time to lock onto the vehicle.