I may disagree with his methods, but at his heart, Gevlon is right. Most people do not play anywhere near their full potential.
Why then is it verboten to point this out? Maybe all the people who advise you not to make a fuss are just cowards. They're afraid of being called "mean" or "elitist". Who cares about about other people, so long as you get your badges. Much easier to take the safe route and avoid confrontation; to laugh about a poor player in guild chat behind her back.
Surely the better path, the harder path, is not to turn a blind eye to poor players, but to help them become better ones.
The difference between Gevlon and I is what we believe are the motives. To Gevlon, bad players are "morons and slackers", who are bad players because they choose to be bad. In my view most people want to be decent players, but a lot of them don't really know how to be good. They don't see the path from what they are to what they could be. Or worse, they ascribe the difference entirely to gear, which is pretty much the worst mistake you can make if you want to be a better player.1
Pointing out that a player has poor DPS is not mean. It's a fact. If you can shame or push them into seeking out external information at Elitist Jerks or the WoW Forums, that's pretty much the only thing which will make them better players.
Of course, though, it's all about style. Calling them terrible and trying to vote-kick them from PuGs unless they do better isn't going to work. It would work if they were actively slacking, but not if they don't know how to do better.
1. See the Why Are DPS So Bad? articles on the sidebar for more on this.