There's fanfic out there that would put published novels to shame. There are also published novels out there that put both authors and publishers to shame. 'Fanfic' isn't an insult. It's not all written by 12-year-olds any more. And getting something published isn't always a sign of quality.To me, it's glorified fanfiction.
Also, tie-in novels can be excellent. Una McCormack's prequel for this show certainly is!