I agree with Pidesco. If the screen isn't shared, then the co-op "experience" would be unwieldy.
I was really looking forward to Sacred 2 before it came out. After playing online co-op for a few weeks, I just gave up. Sure, it was nice being able to go wherever I wanted, but the gameplay was so disconnected from my co-op partner that I just lost interest.
Yes, having everyone on the same screen, especially with four players, can get crowded. But allowing the players to split up and wander independently is worse. Without a plot to keep the game moving forward, I'll lose interest. It's a hack-and-slash, so the gameplay isn't going to change a whole lot, especially if you keep separate from your party.