Feo Takahari
Auror
I think any kind of writing can be good practice for writing skills, so long as you put as much effort into it as you'd put into publishable writing. I also think it's a mistake to start writing based on publishable ideas when you haven't had enough practice to write those ideas with the quality they deserve. Fanfiction is one of the two unpublishable types of writing that's guaranteed to have an audience on the Internet, and while I myself went for the less savory type, I can understand why some people would prefer fanfiction to that.