Gremlins aren't particularly strong, but they still can be a nuisance.
1) They can ruin the entire game for Deleb and Vittorio with their ballistas.
2) They can sabotage the catapult.
3) They can repair (retraining isn't that expensive).
4) With mini-art, they become incredibly annoying.

P.S. Someone said that demons are weak creatures...
Let's see how you'd feel when your mana is depleted, or when their summoned squad blocks your archers, or when they take down a couple of your high-level units with "Hellfire."