Why do people PvP in a MMORPG? If you ask the fans, you will end up with one variation of the answer that fighting against another human is intrinsically more interesting than fighting a scripted AI mob. Which is pretty much the reason why I keep enjoying World of Tanks after over 3,000 battles. But if we observe what people actually do in a MMORPG, does this answer hold up?
It is known that given the chance PvP players in a MMORPG will engage in win trading. Recent examples are a rift PvP exploit in Rift or Tol Barad in World of Warcraft. And then there are countless stories from pretty much every PvP MMORPG in existence about how players went to extreme lengths to win PvP by means which have nothing whatsoever to do with actually playing the game: Denial of Service attacks on the enemy alliance's Teamspeak servers in EVE. Attacking the enemy keep at 3 am in the morning when all possible defenders are logged off in WAR or DAoC. And the list goes on and on. If people were actually interested in fighting a more intelligent opponent, then why would they trade wins or attack when only a token AI defending force is present?
I think the answer is that in a MMORPG the ultimate purpose of any activity (PvP or PvE) is always character advancement. And that character advancement purpose has the unfortunate habit to trump every other possible purpose. Frequently to the point where people choose advancement over fun. "Yeah, it would be more fun to actually battle the other faction over that PvP rift, but lets just trade wins to get more prestige per hour." Unless game developers succeed in the impossible task of every activity giving exactly the same amount of reward for the same effort, players will always look for shortcuts to faster advancement, regardless of whether that shortcut is actually fun to play.
The solution is to rather do PvP games which aren't strictly MMORPGs, and where the fun of the battle against an intelligent opponent beats out the draw of the character advancement. I've found mine with World of Tanks. I hear League of Legends also works that way. And Trion goes from Rift to End of Nations. I think we will see a lot more successful PvP in the future that aren't MMORPGs.