So wait, the arenas that are scattered through the world that the game has special map markers and huge signs and stuff for are there so you can fight the people in your co-op team? You can fight them anywhere! There has to be something more to it. Tell me there is something more to it.
Maybe you can get exp or money from the actual arenas? Come on!
Btw, the game really ramps the difficulty up after you get to Old Haven. The crimson lance are the first npc's I can't one-shot.