They'll never be helpful to you personally (and that's ok), but I actually find people's rude and disrespectful comments helpful (in the same way I find their "appreciation" helpful.
I find professionally that exceedingly positive comments are just as useless as exceedingly negative ones, but at least the positive ones don't ruin your day.
I think the point folks are trying to make isn't whether it's right or wrong to make rude, inflammatory comments. I think the thing people are trying to express is that posters aren't doing themselves any favors by posting nasty comments.
Oftentimes a person may have a perfectly valid criticism that the artist would benefit from being made aware of. Unfortunately, it's human nature to only register things that make us angry and to disregard the rest. Consequently, by basically telling the artist that the piece sucks right off the bat, any valid point being made will be lost amidst the seething fury that the one comment invoked.
This isn't even a matter of being thick or thin-skinned either. I've seen many people make perfectly valid criticisms amidst long-winded posts that essentially told the artist that they were worthless and needed to stop doing art. I'm not sure how one could make a logical argument that it was terribly helpful to the artist, in that the artist probably stopped reading it halfway in. (Kind of like this one, probably).
To illustrate my point, imagine that in real life a person passed a car in a parking lot with the lights left on. They happened to know the owner of said vehicle and decide to let them know that they had left their lights on. Logically, the easiest thing to do would be to simply walk up to the person and tell them that they had left their lights on. Unfortunately, from some of the posts I've seen here, A number of folks on this site seem to believe that the best way to do this is to walk up to the person, hit them across the face, insult their skills as a driver, and THEN inform them that they had left their lights on. Sure, the car's owner has technically been informed that their lights have been left on, but I doubt very seriously that they'll even register that data while they're laying on the ground massaging their jaw and telling their friends what an a-hole that other person was.