Why is a Necromancer not a good choice?
If, for example, you play defensively. While demons are summoned from the gates, lay mines. Then, more mines, summon fire elementals, and a phoenix (on your territory, moreover, many take imps, it's clear that we'll see whether to summon a phoenix or not).

The number and defense of units + the hero's high magic skills allow you to transform a demon hero. And spiritual connection actually helps a lot.

Besides, raising the dead is a summoning magic; take Vlad and use summoning.

On the other hand, mass slowing and weakening of demons is very tempting, but we're talking about a battle against Marbas, and in this case, I'm sure that switching to summoning is justified.