Where Ewan Morrison fails, IMHO, is in working out why we write fanfic. He seems to think it's to get our rocks off or an attempt to 'own' our fictional heroes, whereas for me and I imagine most other writers it's about what might have, could have or should have happened, in a version of our favourite reality unconstrained by budget, copyright, the need to fit everything into a forty-minute episode or the series getting cancelled unexpectedly.
Thus the Doctor Who Missing Adventures are fanfic, as far as I'm concerned, despite their official status, because they provide sequels to televised stories and explore past Doctors from a modern perspective.
Furry fic is fanfic in its way, fanfic about a fandom (metafanfic?), and a lot of the authors now writing about original anthropomorphic characters probably cut their teeth on TaleSpin or Thundercats fanfic. It's as galling when a mainstream novel with anthropomorphic animals is praised for its originality as when this praise falls on an ascended Twilight fic, and when media attention on the genre inevitably focuses on the sexual aspect.
Incidentally, I had no idea that Douglas Adams had a stance on fanfic but I bet he found the idea interesting. I suppose it had occurred to me that there was H2G2 fic, but I'm scared to examine it too closely in a universe which already has a canonical planet called Eroticon 6.