So skeletons suck, and they do depreciate rapidly, but I'd definately rather have one unit that is stronger get raised, then to have two, there is a ton more micro involved. Not that I can't handle it, it's just a preference that I have.
If I see a group of undead headed my way I skip the hero and go for the necros first... then go to the hero, Necros are a pain in the ass. That's my micro for the situation.