I think all skills are useful for writing. Especially creative skills, but it also doesn't hurt to have a good grounding in some field or some activity or some anything if you end up wanting to write about it (or even if you don't, you may be able to translate the experience sideways somehow).

On the other hand, no skill translates exactly, and your comment about roleplaying reminds me that I pretty much consider roleplaying to be the opposite of fiction writing in a lot of ways. (Probably because I do quite a bit of both of them, so the differences start to stand out.) I do agree that it's good for developing certain writing skills (especially voice), but it frequently seems to me that the things you have to tell a collaborator are completely different from the things you have to tell a reader, so in that way roleplay writing comes out totally different. And often the amount of tension I want in a game is different from what I want in a story.

Of course, this might have a lot to do with the specific form of roleplaying I do, or what I want out of roleplaying as opposed to writing.

And it does make me think about writing a lot, when I'm roleplaying.