I think there is sometimes a quality of mercy in being the one to say, "Hey, uh... that's not going to work," but that it's the sort of thing one shouldn't say unless absolutely sure. (For instance, when someone's preparing to attempt something that appears to be impossible no matter who's doing it, which takes the personal-criticism angle out of the equation.)

Just issuing blanket statements like "You can't do that, because you can't do anything," though, I fully agree that's just gratuitous general douchebaggery.