I'm sure they could, but given they need someone to summon them it's a bit pathetic. Espescially as there aren't anymore Cetra. The planet's lifespan is ridiculous compared to that of a species, it would't commit it's defences on the off chance it was attacked in that tiny fraction of it's life.
I still don't see why the wepons weren't sent North, between them they should have just as much power as the Sister Ray.
