It's simple really. While WoW did copy systems from other games, they did managed to bring them together and improve on them in a number of ways to make them very successful.

Older MMO's tried to make a wheel, it came out like an octagon. WoW took that wheel and refined it.

Why not use these systems in other games? If WoW can get away with copying the original, why can't other games use a version of WoW's system?

Who cares if "Game-X had this first", or "Game-Y had this first"; if we enjoy our games, what difference does it make where things come from?