The most USELESS monster.

Well, in that case, you can take 2-3 Imps into your army, no more, or summon them if you have Demonology. They steal mana in any amount, and for some reason, it's always the same, and they're not really useful for anything else.

Well, I wouldn't say that, they take up a slot! So if you're going to take them, take more, so they can at least be used as cannon fodder. Here. Or summon them.

As for their uselessness, just the other day, as far as I remember, I used those same Imps to hold back the last remnants of the enemy while I bombarded them with magic from a corner. That's where they were especially useful!