There are multiple reasons we wanted mob spawning off in larger towns. TPS and lag issues are, and will be an issue outside of fights if large numbers of big towns are loaded (who all have mob spawning on). This is a problem that will only get worse as the server grows, so the change helps to future proof us.
The second major reason was to encourage people out of their towns. People often complain there is not enough wilds PvP and a big reason for that is people sit in the safety of their town all day. It also doesn't really make much sense for a big thriving town to have a bunch of undead and mobs running around everywhere.
Finally mob grinders and dark rooms. These are things we ban on Loka because they pretty much nullify the mob side of PvE. Especially when we have plans to add custom mobs with cool unique drops. They also kill any economy or trading for mob drops. We have been finding increasing numbers of players/towns ignoring these rules and hiding dark rooms/grinders within their towns, sometimes in plain sight. This is something that is much harder to do outside of your town and much easier for us to catch those doing it.
To find the best solution I think the big question is why do you want mobs spawning in town? Is it because mob spawns are not consistent or good enough in the wilds? Or something else?