I disagree on your argument that saying 'be polite you jerk' merely drags things down, for two reasons.
First of all, some people don't notice their behavior is unnecessarily impolite. Pointing it out can help them (if they are willing to be reasonable in the first place). Never pointing out somebodies failures will make them fail forever.
Second, it shows you care about being polite. If others show they care too, a culture of 'you should be polite' can be maintained. As you might have noticed from the differences between FOSS communities, culture is important and heavily influential. And it can be changed.
Some things to note:
- people DO care about what others think of them. No matter how much they scream 'no I don't', they do. It is our nature.
- people should know their arguments are not supported by being mean - it is the other way around.
- I agree that a 'be polite you yerk' might not always be the best way to correct someone. A personal mail can do more. However, it won't show up in public (unless an apology is made), thus it does not much to influence others who might think it is acceptable behavior because the guy got away with it. Of course, giving a good example is better than anything else.
- Of course discussing without end whether somebody was polite enough or not muddies the discussion and lowers the SNR.