Mostly because it's limited. It's effects aren't out of whack, permanent, or unique...it doesn't increase the quality (the power!) but does increase the quantity (more power!), meaning a DM who is prepared doesn't have to accomidate for anything new when magic items or spells are invloved...it's all the same thing....all spells, with the usual thwartings involved.
Those are the main reasons that spells and magic items are okay...and I'm still not *entirely* comfy with it, but I'll deal.