AlKim: I can understand the time limit, because otherwise you would be able to turtle the inevitable waves of enemies until you can rush with the undead alone. SpellForce 2 seemed like a relatively fast-paced RTS to me as well (although I don't play RTS's very often, so take that with much salt), so I never found the short duration of raising the dead detrimental to my efforts. There was one map where I did rely on it for defending my base until resources for better units had been gathered, which I found pretty hectic and satisfying.
Stacking summons is just bullshit in my opinion, but whatever floats your boat, I guess.
Actually I should have continued my idea.
I rather have a limited number than time limited summons. Because then I'll have to make use of a small numer of units against a larger number units instead "rushing" before they disappear.
In SpellForce as I recall if you can have a summon it drains a part of your maximum mana but it's not limited to a time duration, only until you unsummon (I've a vague memory this was possible) or your enemy kills it.
Of course different rules should apply to different genres and I was talking mainly out of a typical RPG scenario as opposed to a strategy game where armies is a big part of the game.