Late-game Town Enhancement.

Maybe it's because in their own city, a Warlock won't be able to learn Darkness, even at level 3? And besides, why would you need Darkness against Necromancers, who are immune to mind control, Berserk, mass Dispel, Plague, and Vampirism? Taking Darkness just for mass Slowdown doesn't seem very sensible.

Well, why immediately Necromancers? There are plenty of other options in 3.1, not everyone plays as Necromancers. Besides, there's Leto, who, let's say, has dark magic by default. It's not very effective against Necromancers, I agree, but a slowed-down Necromancer is great; by the time he figures things out, only skeletons will be left. And what about Chaos + Summoning? I think if you think about it, you can use it effectively, although it's two schools of magic... anyway, it's not a given, I'll try it...