The most useful monster.

But of course, I'm not building absolutely everything... but most of it.

You're just making things harder for yourself.

and skeletons are good in large numbers (thankfully, necromancy was created for that)

There won't be much fewer Imps. Necromancy is designed to raise vampires, or at least ghosts, and skeletons are just a short, intermediate stage. And it's sad to run around with them because of their low movement speed.

So the choice of units depends on what you need... at the moment.

But some units are needed in many situations, while others are just a temporary addition. Therefore, some are more useful than others.