Even with the ease of getting such corpses, it's still a pain to get the zombie army ubless you zombie Lord and master animator.
The spell has a high failure rate, especially if not mastered.
Even with both those edges, you'll still destroy the corpse occasionally. While admittedly the water sprite was too easy to kill and raise, the rest pose much more risk, especially at the lower end of levels.
And if you're not careful you lose the ability to fight those high level mobs for what, 70-80 In-game hours? (Speaking wall of putrid flesh here.)
Yes, once they have that army set up, they're very powerful, but before they have that (about an RL hour after they log in, minimum.), they're almost easy PK's. Provided you don't get slept.
Although I HATED when my zombies would be summoned away by out of range paladin/shaman/whatever, I'm fine with the current system. It makes them truly feel like what I believe a necro should be, and I personally don't want that to change.
Then again, this is all just my opinion.