I wish it went like this:
Person 1: Speaks idea and justifies theory with incidents and trivia from canon
Person 2: Speaks contrasting idea and justifies THAT theory with different incidents and trivia from canon
Person 1: Continues to support their idea by providing more "evidence" from canon and possibly the real world
Person 2: [etc].
...with mixing in from other people, etc.
Instead, what happens is that people can't bear to hear ideas supported that disagree with their own. They take it as a personal *insult* that someone would have the nerve to actually *support* a dissenting idea with evidence. (It's apparently bad enough to disagree with people, but to actually try and justify your idea is unforgivable.)
OR, there's the bizarre viewpoint that since all fanfic deviates from canon in that it's (by definition) not canon, that means we can't use canon as a jumping-off place and a limiting factor. In other words, if you point out that someone's out of character in a story or that a particular incident is contradicted by canon, the reply is just that "all fanfic deviates from canon." That's not exactly how it works. Fanfic, for the most part and unless it's AU, is extrapolated from canon, not deviated. (I hope that made sense.)
Saying that we might as well ban all fanfic for being out of canon because someone pointed out that a story was unreasonable or out of character leaves us with no room for critical discussion of the text.
shatfat and I have a tremendous amount of respect for each other, yet we disagree about canon sometimes. It makes for great discussion!