Karlach, Halsin, Jaheira, and Minsc should never have been companions IMO. There just wasn't enough time to flesh them out before launch (they were all initially intended to be just quest NPCs). I applaud Larian for trying, but I feel it was a bit too ambitious.
Let's hope they continue to work on them (and Wyll) for the Definitive Edition.
Tbh, I find Karlach and Jaheira both a delight and am glad, they are in. But yeah, Halsin and Minsc are unnecessary. I mean, they made Minsc and Boo entertaining and fun to have around, especially his dialogue with Nine Finger, but he comes too late to actually make sense. Jaheira has a lot of content and dialogue in act 3 actually.
Halsin, well I said my part about that already.