yeah, because monsters tend to attack people. you would think that a group of trolls might want to attack a settlement and run off with some people to have for dinner every now and then. after all, if the world has gone to hell in a hand basket then food should be scarce for them as well. they should get desperate and attack settlements.
this would go along way towards reducing the population of the settlements. so that you need to protect them from roving bands of monsters to keep your population up. every now and then they also destroy an improvement as well.
it seems to me that in beta 2 they have gotten rid of the monster lairs. i don't know if in beta 1 the monsters spawned from there or not. i have seen some monster type locations, but i think a single monster guards it.
creatures should have their own faction AI controlling them as well. they could have 3 types of AI.
Passive: monster tries to keep to itself for the most part. this would be similar to mundane animals.
Defensive: monster attacks if units, settlements or other threat comes within their territory.
Offensive: these creatures seek out targets and attack, they are predatory, or conquerers.