Me and Gabez have already had a bit of a chat/argument about this. I'm on the side of 'angry Adams fan'. I don't think this book is necessary, and given the choice I would've much rather had another movie. Adams is just too unique to even attempt to copy.
Strangely enough, I don't have quite as much reservation for the third Dirk Gently radio series, which is pretty much doing the same thing. The difference is that Adams was working on Dirk Gently 3 for years, and there's a lot more written than was published in The Salmon of Doubt for Dirk Maggs to go on.
Let us also not forget that the only book so far based on an Adams work but not actually written by him is Starship Titanic by Terry Jones, which was awful, despite Jones usually being an excellent writer and Adams still being alive.