I am in favor of MP for DA:Inquistion, specifically co-op MP.
A DA version of ME's version, of GoW's Horde Mode is the first thing to come to mind, but co-op campaigns are defininatly possible (and desired), I would ask for no PvP though. PvP tends to have a lot of negitive trends associated with it compared to PvE (from my experience).
The biggest negative side effect to having a player vs player layout is it inevitably only appeals to the hardcore power gamers, people who have the dozens of hours available everyday to hone their skills and become godlike death dealers on the battlefield. I can tell you, an inexpericenced person is only going to tolerate so many games where they have a 0:50 K/D ratio, before they quit in frustration. Plus, the balancing needed for PvP is alot more vital then PvE, which will lead to the classes/races being simplified for the sake of balance; thereby destroying the unique flavor that was originally present. I personally don't have any desire to play on the blue team, fighting the same types of classes/races on the red team.
MP should (IMO) be something that you can do with your friends that you could not do in SP. Granted, the fact of playing with other people is something that can't be done in SP, but I am refering to things like race/class selection, and weapon/armor choices. I would love for MP to be the place where you can have a Qunari Templar questing alongside a Sylvan, an Elf Blood Mage, and a Golem. A place where playing as a human rogue who uses hand to hand attacks rather than knives, a shape shifting Mage, ect. That's what I would like out of my MP.