It would make griefing easier and, anyway, if you want to build something big and impressive, it's not exactly the best idea to build it on a creative server.
Also, it would probably increase the amount of laggy plots.
We can't just give it randomly to the hundreds of people who come on each day, creative already lags enough WITHOUT people changing big chunks of blocks at once...
Using world edit will mostly (and usually) create client-side lag. So if your world-editing isn't being rendered by their game, they probably won't have much lag at all. Buuuut, this is just a theory.