As far as I'm concerned a good crit is a good crit even if the person giving it can't draw for shit. A lot of people have better eyes for critique than hands for drawing. If a piece of advice helps you address an error in your work you should be happy to have gotten it, not think in terms of 'well DO BETTER YOURSELF THEN!!'. It's not of direct interest if they can or cannot, it doesn't change that you have the error in your work to address yourself.
Also the 'he talks as if he's 100% right' is not directly related to the quality of critique either. You have a brain of your own to make your own choices as to how useful a piece of critique is. That being said I can see how such attitude would grate, but let's be clear it grates because of our own insecurities. And THAT being said such behaviour should probably be moderated in a critique board because a positive attitude, not an antagonistic one, should be cultivated for such a place so as you see we would give notices and warnings to people on attitude in Pixelation. Just keep in mind that on the psychological level, the problem is more with you getting riled up by such attitudes than with the people having the attitude. They might have it because they're conceited jerks that indeed think they're always right, or they might just have no time for pleasantries and just want to help as directly as possible and trust you to make up your own mind as to whether they're right or not.
It's been proved though that if you're super-nice and pepper your critique with more 'that's just my opinion though' s and etc the person getting critiqued will take the harsh crits more lightly at first read. That's both good and bad because a crit taken lightly is a crit lightly dismissed also. And besides, just a scathing, harsh, super in your face mcblunt piece of critique that makes you fume at the first 5 reads of it... well you can read it a sixth time eventually and learn from it, it's on the internet in written form, it's not going anywhere (as opposed to real life verbal crits where being super polite is even more important).
My own reasons for critique have been mentioned a few times and they're pretty benevolent: I get a huge kick out of seeing someone be happy over improving their art because people helped them out of their own good will: it's a small miracle when it happens. Also it helps my own art a huge lot to be critical and analytical of other people's art.