O what's that? Dont have enough mental resistance? Well in that case you are going to be locked in Crushing prison. Mages have the advantage of being range and unless you have a way to quickly reach the mage, you're done. Crushing prison, based on the previous two games doesn't have a time limit until it wears off, meaning that if you don't pass the mental resistance check, you're going to be locked permanently
This is exactly why there will be no PvP. I hate it when Mommy and Daddy fight!
Super-cereal though, the reason this will most likely be PvE only co-op is the DAMP game mode is meant to simulate Agents of the Inquisition. Unless the Inquisition has real internal cohesion or morale problems, because your Inquisitor is a really poor leader, I can't see the agents fighting each other to the death constantly, can you? It would also break the serious tone of the grave threat facing Thedas to have an endless internal 4v4 scrimmage game running in the midst of the Inquisition's ranks. Either way, DAMP seems "seated" thematically within the larger narrative of the Inquisitor's story and would need, for me anyway, some type of plot-related justification to incorporate PvP. If the game included a mode which would allow you to play as the Inquisition's antagonists, I feel it would divide the sympathies of the player between emotionally identifying with the Inquisition and its enemies. This is not to mention the Class-balancing issues from a game design standpoint which would need to be addressed, as others have mentioned.
However, in future games it could be cool if the protagonist's affiliations were more open-ended or you were playing as someone who must choose to join one of a number of competing organizations you do not control (and all the classes were balanced) to implement PvP... But mostly because I hate it when Mommy and Daddy fight! It's scary!