We're discussing two different things here, I see. One I can comment on, while the other I can't, really.
I'm not an author, can't write to save my soul. So I can't really say whether writing in another's world would be helpful in terms of getting better at, well... what? Writing? I can't imagine it wouldn't. If your goal was to get better at world-building, then obviously you wouldn't get much practice if you used someone else's base. But if you were just practicing writing, I don't know why it would be any different - in fact, I imagine it could be
harder in some ways, having to adapt your own writing style such that pre-existing characters spoke and acted in ways that wouldn't break existing characterization.
As a voracious
reader, though, I can say this - most fanfic sucks a lot, to the extent that I really don't bother with it, as it's mostly a waste of time. But that's not the fault of fanfic itself, it's merely the fact that the genre attracts far too many people who write about as well as I do, but have much bigger egos
. I
love to see different takes on the same stories, or the same characters, or the same worlds, but only when they're worth bothering with. If I
were a famous writer, I'd be flattered by any and all attempts at fanfic in my world - even though I'd be even more saddened at how much most of it would suck a lot.
Incidentally, I absolutely
love covers; in fact, I go out of my way to collect them. Have gigs and gigs of it. I've also
deleted gigs and gigs, because again, I only keep ones that are actually worth keeping. It seems that fewer people with little talent consider themselves to have talent in music than it writing, though, so it's at least worth bothering. Or at least, less of the music that sucks ends up being discussed and passed around the internet.