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It's a pretty vicious cycle..idk about WoW, but Rift you can play all the time..
The more you play the better soul points you get. The more soul points you get, the better aspects of your character you can unlock. Plus there is always pvping going on and even if you don't win you still get points for the some of the best gear in the game. Which takes a LOT of time. Dungeons get old after a while, but you also get currency to get you the second best gear in the game next to raiding. Raiding is done with your guild and sometimes can take a few hours a piece depending on the skill level of the guild. Then there are events to get more currency which gives you special items..
There's a lot to it..but the people that spend all day playing definitely have an upper advantage then the casual people.