The problem with a co-op mode is how it will effect the final game. Larian would either have to do two different versions of the game (which they don't have the resources for), or make restrictions to the game so that it will work as either a single player or multi-player game.
I suppose the easiest way to allow for multi-player without compromising the story or quest depth would be to have one person playing the main character, and any other players as secondary party members who can not get quests, etc. Alternately, any party member could be allowed to get quests, but they would effect everyone at once (ie once a conversation was started, all players would see that until it was finished, so there would be less chance of conflicts).
Either way it would add complexity to the game, for a feature that may or may not be well received by most people in the form it would have to be implemented in.
Welcome to the forum. 
 
 I see your point and I agree.
As I said before, I really don´t care if the co-op players doesn´t have a main role. They could  be only players that do everything, but are not part of the history. As long as the co-op players can level up, get armors, buy and sell, I really wouldn´t care  (it´s better playing coop this way than not playing coop). BUt if the devs have not enough resources to allow more than 2 players playing simultaneously, while keeping a decent framerate and nice graphics, then I will understand the devs.
I really would like to play this game in coop mode. Think with me: at least on consoles, we still don´t have a game like that with decent coop mode... Two Worlds? Meh! Kingdom Under Fire? Big meh!
Coop has evolving a lot these times. I´ve never seen so many games in so many different styles that allow for cooperative play. It is amazing! A lot of people are married, have children or friends that wanna play together.