It's worth keeping in mind that even if she's completely useless for any sort of mission, Bellatrix may still be useful to Quirrell, by providing materials for the spell to restore his original body. This was discussed shortly after she was broken out; the spell would be at its most effective if the ingredients used were the most powerful of their kind; not merely the flesh of a servant, but the most faithful servant, and so on.

No matter how much the story diverges from the original HP canon, it's still an option for characters to do the same things they did in the original, as demonstrated by Quirrell in the last couple of chapters.

MoR Riddle still made his horcruxes, and he didn't do it for nothing.