As of right now, aggressive mobs will not spawn. Passive mobs will (animals) but butcher is often used by the staff to help with server lag. So you might come back online and all of your chickens and cows will be killed off.

I gave up on farming animals in my towns due to butcher being used and now only use the aether to farm animals. Butcher does not seem to be used as often in the aether.

 

You have to pay someone with ext creations as even passive mobs are not spawning in towns with it flagged on. I am sure they are supposed to spawn in towns with it flagged on but I have 2 with it and I haven't had a passive mob spawn naturally in either one since hostile mob spawning was disabled.