I don't care a whole lot when other people use what's on Rowling's website as canon, but when they talk down to me—for instance, in a review—when I don't use what's on the site or when I use something contradictory to what's on the site, then I get a bit pissy, missy. This fandom comes in many major forms (books, movies, games, the website), and because of that the boundaries are vague, which means we must make personal decisions on what we consider to be the ultimate source(s).
Since I am on the subject, I'm of the opinion that the movies and games cannot be considered canon because they are, in a sense, fan fictions themselves. They are interpretations of an original source, even stray from it on occasion (and because of that it would be difficult to take them into account while still fully taking the books into account). They were not created as extensions of the stories but as showcases of them. They revolve around the stories, but they are not the stories themselves.
As for whether I consider the website canon:
If an artist drew a picture of just a box you would not know what was inside the box. You would be free to interpret its contents yourself. But say the artist came along and told you there was a sheep in the box. Would you be required to believe him just because he told you so? No, because he didn't put the sheep into his artwork; therefore it is up to personal opinion what is inside the box—until the artist actually draws the sheep.
(Excuse me for butchering Le Petit Prince.)
Similarly, there is information on Rowling's website that has not yet made it into her literature; so, until she actually uses it, it is up to personal opinion whether or not it really exists in the Harry Potter universe. Therefore: not canon, not yet.
I'm wondering, whether or not we agree on our boundaries of canon, why some people are so persistent about their own. I have my own boundaries that I like to write from, but it would be silly of me to tell another author why her fic is wrong because she wrote about Legolas-hair!Lucius when I prefer him with short hair. And it would be silly of someone else to tell me I'm wrong because I wrote that Ginny's name was Virginia and not Generva or Genivra or Genderfuck, or whatever Rowling has introduced.
Maybe I'm alone, but I've gotten several reviews on such subjects, and it's quite tiring.