You're right, the game isn't necessarily competitive, though with some of the dialog options and role-playing elements I don't think you necessarily need to play completely cooperatively either.
So why does it matter if the game is unbalanced? At least part of the fun of any RPG that allows character customization is creatively experimenting with abilities, stats, gear and play styles to find effective combinations. Not all of it, for sure, there's still fun to be had by taking part in the story, reading dialogue, etc, but I think by making these kinds of choices easy and obvious as a game developer you do end up taking some of the fun out of your game.
Consider an imaginary strategy game, let's say it's single player so balance again isn't REALLY a problem since it's just for your own enjoyment anyway. Let's say in this hypothetical RTS there are dozens of units with potentially interesting uses, but there is one unit that is cheap, builds quickly, has insane health, damage, and range. It can be used for anti-air, detects stealth, and moves quickly. It's really the only unit you need, and try as you might to find one you don't have a reason to build anything else. Hopefully you can see how that would suck some of the fun out of the game; the game strategy has now been simplified to the point of being well... pointless.
Simply I would argue that mages in Divinity are currently that unit, and although you could potentially create a viable party without them and deliberately handicap yourself, doing so would mean that I as a player am challenging myself rather than the game challenging me.
And thats wrong. Mages are not too strong. They are perfectly fine. The Problem is again Lonewolf + Class Cannon = 18 Ap/Turn = much Destruction. But that is true for every Class. In out First Playthrough ( Beta ) we played Warrior + Rogue. Still easy and there was no Lonewolf + Class Cannon. Only Lone Wolf back then. Warrior one hits everything that comes close to him and Rogue assasinates everything else + Special Arrows are really fucking strong. Even a Party of 2 Warriors. Bull Rush in -> Whirlwind -> Mass Death = GG