Wilhelm is asking if an MMO must contain PvP. It’s actually a more interesting question when you really thing about it, especially if you limit the discussion to themeparks (the answer for sandboxes has its own tab on this blog).
Instinctively you might want to say ‘no’ for themeparks, because they are PvE focused and you would want that to remain the focus. Makes sense, on paper. But in reality, themepark PvE content is often one-and-done, and what is repeatable (daily quests, raiding) is often tied to some long term, but still one-and-done reward (rep grind, raid gear).
PvP shouldn’t be the focus, but rather play the role of filler between content updates. From my experience I think vanilla WoW did this best. Whether you were waiting for a raid to reset or had a night off, battlegrounds provided a nice side activity, made more rewarding as you could use your raid gear to get an edge (though not a brutally overpowered one from MC/BWL gear).
As time went on PvP in WoW got a bit silly, first with PvP-specific gear and later with rankings and all that stuff. It went from being a fun side activity to a game-within-a-game. It also didn’t help that all of the talent on the WoW team left and the interns ran the place, but we know that story.
I like, on paper, what ESO has planned. Once you reach the level cap, you can still PvE to gain more skill points for horizontal progression (you can only use a small number of skills at one time, so getting more skill points to open more skills doesn’t increase your power, just gives you options), but you can also get into the 3-way RvR battle areas. I think the limit to horizontal progression will help the PvP balance a great deal, as will the fact that (as of now) the best gear comes from crafting, not PvE, raiding, or PvP. Assuming that stays mostly true (a few items being BiS from non-crafting is fine IMO, so long as most stuff comes from crafting and the gap isn’t too great), I can see the model working.
I can further see it working because as players spend time on the repeatable content that is RvR, Bethesda will be given time to expand the PvE offerings. I don’t think ESO players will experience running into a ‘content wall’ like in SW:TOR.
So my answer to the question is yes, you do need PvP, but at the same time you need to ensure that the PvP remains a low dev time, high repeat, limited impact aspect. Not easy to get right, but certainly pays off if you do.