Even if it only works for a single attack or two, it would still put the zerg in an advantage, as the orks lose the remains of their boyz on the creep, while the swarm gains them. As an example, say the zerg setup a fast creep build then attack. They start out 50/50 evenly matched with the orks, but after the battle they manage to hit 60/40. They now have a numerical advantage that can be pressed. In fact, that could be a very smart tactic that can be reused in different ways. An ork attack is coming in? Drop a tumor behind the zerg lines and let the creep grow during the battle. If the zerg win, they absorb the remains. If they lose, they didnt really lose anything extra. By holding the battle outside of secure zerg turf, they dont risk their hive in the skirmish, as whatever gets through them still has to deal with the main hive defenses and will have been weakened in the first assault.