"Weak demons"??

Ranged units only manage to fire once, then the summoned creatures from the abyss interfere, and the neutral creatures still need to reach your units. And if they do reach them, you have to actively move around the battlefield, and then: no one has taken away the Imps' initiative or the Cursed Sign from the demons...;) Removing retaliation is far from the only function of summoned creatures: how about luring neutral creatures to the desired location or blocking a friendly unit?
Indeed, losing 30-40 creatures in the first 2 weeks is significant, but these kinds of losses will end then. Afterwards, we lose no more than 3-5 creatures of levels 1-3...