I know that one's supposed to have all three varieties of members in a party, but the thing is: I am a mage but I also like to have Anders and Merill along. For the remaining slot, I usually prefer Fenris. So, thats three mages and one warrior. The battles were always satisfying with the above party but I just want to know if there would be any advantange if there's a rogue except for the fact that they can detect traps and unlock locks.
Don't underestimate the lockpick ability: there are a ton of chests that require lockpicking skill.
If you're familiar with Dragon Age: Origins, you probably remember that the rogue was by far the most powerful class to the point of excessiveness: a single, high-level rogue could outmatch 3 warriors or mages. Of course, a melee rogue companion (like Zevran) was next to useless because you couldn't position him for backstabbing unless you were directly controlling him.
Dragon Age II smoothed the curves, as it were, when it comes to rogues. They're not clearly overpowered, but they still have a lot of bang for very little buck. And it's easier to outsource melee roguing to Isabela since backstabbing in DA2 is auto-positioned.
So you can forgo the use of Isabela (the only melee rogue you can get), Varric, and Sebastian (the two archers you get), but you're probably making the game a little harder for yourself.
Balance aside, Dragon Age II made great strides in trying to encourage use of all your companions:
So feel free to mix it up every now and then.