I'd say it's pretty much up to each individual fan what's part of canon and what isn't. What troubles me personally is Sherlock Holmes published fiction. A lot of authors seem to use Holmes' name as a crutch to get their borderline fanfiction published, and I don't feel that's appropriate.
I once read a Holmes novel entitled "The Seven Percent Solution." I went in assuming that was a reference to the eventual solution to the crime, but it was actually referring to Holmes' frequent use of a 7% mixture of cocaine in water. The novel is a crossover with real-life Sigmund Freud, who psychoanalyzes Holmes (who is a worse addict than he ever was in the original stories) to uncover the mommy issues that keep him a bachelor and led to his substance dependencies. My real problem with the novel is that it implies Moriarty, the admittedly never-developed villain of the Holmes-verse, to be a common math professor; Holmes has subconsciously imagined all his criminal activities, probably because Moriarty was romantically involved with Holmes' mother or something. Freud eventually convinces Holmes to seek treatment for his drug use, prompting Holmes to ask Watson to publish a false account of his death temporarily to avoid attention (which is implied to be "The Adventure of the Final Problem").
"The Seven-Per-Cent Solution" annoyed me because it basically sought to undo part of the established Holmes canon (and because Freud, whose beliefs I hate vehemently, was in it, but that's not important right now). On the other hand, I've also read another piece of published Holmes literature called "The Einstein Paradox," which used the vehicle of Holmes (and Professor Challenger, another Doyle character) solving mysteries as usual as a backdrop for explaining quantum physics mechanics. Despite the educational intent of this book (which was even used as a textbook for my high school's AP Physics class), I absolutely loved reading it, because it remained true to the main canon, and really did read remarkably like an original Holmes story. (I highly recommend it to any Sherlock Holmes fans; the author is Colin Bruce.)
So to finally get around to the heart of this discussion, this extra published fiction is very similar to fanfiction: sometimes, it's good, and we enjoy it becomes it fits well with the canon; other times, it's bad, and makes us cringe because it doesn't jive well with the parts of the canon we remember and love. In those latter cases, we just have to accept that those publications do exist, but that we can choose to ignore them. I don't think it's even worth getting angry about, because, as we, of all people, know, there will always be bad writing. We just have to keep ourselves busy with the good stuff.
My apologies for the long rant.