For a more earnest defense of Minsc, I think he's a really well-executed comic relief myself. I think he's a great Quixotic inversion of the heroic fantasy adventurer, is filled with great lines and puns, and is superbly performed by Jim Cummings. He's not a character to be taken seriously, he's literally there to poke fun at the dogooders by offering a counterpoint that seems to exacerbate all of the boistering, righteousness and epic ambition to absurd and risible heights. If you don't vibe with his style of comedy, that's one thing, but I for one think there's plenty of wit at play with his lines and place in the games, and he fits that Homer Simpsonesque "smart dumb comedy" category rather well.
As a partymember on the purest technical/mechanical front, I can agree, he's not great. He's certainly viable, he's fine, but I don't even feel he was the stronger of the two ranger choices in Baldur's Gate 2 to begin with, if you even want to go with either. But then again, in a game like BG2, you don't need your companions to be min-maxed to hell to beat it even at the highest difficulty. And on top of this, I think that is beside what makes him or any character good. As a companion I value what he's able to add as a character, in terms of story, interactions, how he contributes to the overall experience, etc. In this sense I think he outdoes a lot of the brooding and poe-faced male fighters of Baldur's Gate 2, even without a quest of his own (ignoring the one he had in the first Baldur's Gate of course) - and as for Aeria and Nalia, I'd gladly ignore them in favour of Minsc if I found it necessary as well.