[j]oint-carving thought does not have merely instrumental value. It is rather a constitutive aim of the practice of forming beliefs, as constitutive as the more commonly recognized aim of truth. (WtBotW p. 61)I don't think this can be right. The idea of somebody forming beliefs without any kind of sensitivity or regard for whether they are true is incoherent; this is not so for someone who doesn't care whether her beliefs carve nature at the joints. Suppose one is charged with failing to carve at the joints with her beliefs, and replies flippantly --- so what? --- and maintains her previous beliefs? She might be criticizable on epistemic grounds, but her attitude is comprehensible, even if we do not approve of it. Compare the person who is charged with having false beliefs, and replies in the same way --- indifferently accepting the charge, and continuing to believe as before. This isn't just epistemically vicious; this runs counter to what it is to be a belief. In other words, a truth aim has a better claim to a constitutive connection to belief than a joint-carving aim does.
Here is another difference that should not be overlooked: some instances of non-joint-carving beliefs are absolutely correct to hold. Maybe they're not as good as their joint-carving cousins, but one needn't choose between them. You can believe that the emerald is green and that it is grue. In fact, that's exactly what you should do. And you shouldn't feel at all epistemically deficient for having the latter belief. Compare this to false beliefs: every false belief you have prevents you from having a true one.
Moore-paradoxes show a deep connection between belief and truth; there is a deep incoherence in the idea of accepting: "I believe that p, even though not-p." But there is no corresponding incoherence in "I believe that p, even though the terms in p do not carve at nature's joints."
Whatever epistemic value attaches to joint-carving, it is less central to belief than truth is.