Mutual exclusiveness adds some "real choice" into the game, as opposed to "just maxing out", which gets boring quite fast. With "maxing out", there is no real choice: you can either do it right, or do it wrong, but honestly that's not a meaningful choice.
Rolling a character is horrible, but you can reassign the points, so you just hit the "reroll" button until the sum is good (that takes just a minute) and then you redistribute the points as you see fit (which is a bit silly -- and that's the consequence of using a bit silly rule system).
Otherwise, I find the BG quite MM-like in nature: you have a lot of connected maps and you can go pretty much wherever, so I wouldn't say it's not open. (And, by the way, the combat in BG is far better than the one in MM, though it has problems of its own.)