This ability is far too OP. It essentially makes waaagh towers invincible to all but the heaviest ranged firepower (say... 4 lascannons would take it out). This makes fighting the towers nigh impossible, where the towers themselves are already potent.
The whole point of the ability making the tower take extra damage is to allow the opposing player to have a chance to stop it, and to stop aggressive use of WAAAGH (say, activating it right in front of your army). It completely nullifies this drawback of the towers.
Instead, players will activate it in the midst of a pitched-battle, since nearly nothing can kill it.
This doctrine (almost ubiquitously taken) makes attacking an Ork encampment from hard to "not-gonna-happen."