I think that most people who are confrontational and critical online are just like that in person. People can be quite blunt face-to-face, and are perfectly OK with it when people are blunt to them. Blunt interaction has the big advantage that you never have to second guess what a remark means. Consider how many relations (between friends/colleagues/etc) break up because somebody reads too much in a simple remark. The most tactful people also tend to be those who (needlessly) worry the most about what other people say. I say this just from personal experience, so YMMV - but it's worth considering.

As for the Mr. Spock argument, I think that has a much more mundane explanation. I, like most people, read far more on the internet than I write. But when I read I read to learn. Therefore, I want to be reasonably sure what I read is correct. Because of the Mr. Spocks amongst us I can read the comments and use that to figure out if the blogger made any sense. More often than not the comments reveal serious flaws in the blogger's post. It is this interaction between blogger and Spocks that leads to better understanding. Both for visitors like me and the blogger.

So the Spocks fight against the pollution of the internet by incorrect statements%. It's a futile fight, but I believe it's a noble one. Frankly, I think posting uninformed and wrong comments is far more inconsiderate than pointing out to all future readers that such a comment is uninformed or wrong.

%) You called it an internet maxim in another post.