I've used it often in PVP, but less in Legion and WoD than before, seems in Legion mana is infinite even when in PvP. Ah well, as usual I am a minority in WoW
If you want PvP in a game like most of them, you'll need factions to drive it, though you can work around that I'd imagine via merc companies or something else. Not sure that would really effect much of a MMORPG's death, though. Is it a problem to just play the same faction as your friends and just have alt for the other side ?