I think that all writing is useful for honing writing skills. I think you get better as a writer by writing, and whether that means that you’re writing a singularly deep and moving novel about the pain or pleasure of modern existence or you’re writing Smeagol-Gollum slash you’re still putting one damn word after another and learning as a writer.
(I just made that up. I imagine it would go something like: “Oh, the preciouss, we takes it our handssses and we rubs it and touchess it, gollum….no, Smeagol musst not touch the preciousss, the master said only he can touch the precioussss…. bad masster, he doess not know the precious like we does, no, gollum, and we wants it, we wants it hard in our handses, yesss…” etc etc)