if you accept the expression you accept that the subject is wrong by definition

I don't agree. There is such a thing as being unnecessarily pedantic, but there are situations where pedantry is very much called for (say, reviewing the specifications for a communications protocol to be used in nuclear reactor software). There is such a thing as being unnecessarily sexually explicit, but there are situations where being sexually explicit is very much called for (say, while having sex).

I do agree that saying "unnecessarily X" carries a suggestion that X is usually not a good thing in ordinary circumstances. Personally I'm quite comfortable saying that about rudeness (while vigorously agreeing that there are situations in which rudeness is called for); do you disagree?

that politeness is not an unalloyed virtue and that being rude can be useful

I very much agree. I would guess that Viliam does too. (We may not agree on how often being rude is useful.)

Greg Cochran

Yeah, I'd classify that as needless obnoxiousness, and it certainly doesn't make me think better of Cochran, think worse of his opponents, or think him more likely to be right. More specifically, if I compare my mental models of (1) someone who holds Cochran's opinions as the result of impartial scientific investigation and (2) someone who holds Cochran's opinions partly because he's always been inclined towards white supremacism and is glad to have some scientific-looking backup for it, #2 seems distinctly more likely to write that sort of thing than #1, so I update Pr(Cochran's opinions are not substantially influenced by prior prejudice) down just a little.

I doubt Cochran cares what I think of him, and he may well have sufficient reasons of his own for being obnoxious in that fashion. (E.g., I expect it helps to stir up enthusiasm in readers who are already convinced he's right.) But in so far as Cochran cares about persuading those who are not yet fully convinced members of Team Cochran, I guess that that sort of rudeness is counterproductive.