For me, the goal is to reinforce the disipline of writing something every day. Would I prefer it be around a new novel, sure. But I have several WIP at various stages, so being stuck or uninspired is not a valid enough excuse to not write. NaNoWriMo purists may consider this outside the rules, or at least the intent and spirit. Eh, maybe, but I'm not getting to worked up over it. I keep the perspective that a fiction writer sits at their desk and makes things up… so really, how mad you gonna be?