You have PvP quests, which happen in specific zones, meant for that, where people who want PvP go to PvP, without forcing it on people who might not be interested in it. "World PvP" on a PvP server is 99.9% ganking. The moment you take off the ability for people to opt-out of PvP with a flag you cause that, because most people only get into "World PvP" when they are sure they can win. Even PvPers, when pointed out that on a Normal server you can simply leave yourself open to PvP if you want it, said they would exploit that, by only turning PvP on when they could attack someone by surprise.