No Mohdaol, the reason we don't allow the spawning of withers is because there ISN'T a grief defence from it. If one was to spawn, then everything around it will be destoyed.

Or at least, that's what an Admin told me when I last asked about it - they may have found something to protect against it, but I think this would be something done in the PVP world arena or something - I dunno.