Very good point Mark, thank you. You say "does it help anyone?" and of course, it helps no-one. But the crazy thing is that it's often the ranter, the hater, who feels worst of all. How can anyone feel good and happy when all they do is complain?
There are two ways to look at everything so why make yourself unhappy by being negative?
Something I noticed when I was young, was that a lot of older people have terrible attitudes. Now that I've done some living, I realize that life is a wonderful thing, but it beats you up. It is a struggle, to do whatever it is that you want to do. Most people keep the struggle to themselves. And, I think that it is a challenge, and something that you have to work at, not to let life ruin your attitude. So, you never know what your aquaintance had to deal with. But you're right, a bad attitude helps no one, especially the beholder.
Mark -- I have ran into this problem. I've interviewed hundreds of artists -- and I'd say that for every hundred there was always at least a dozen who decided to take a jab at another artist or direction of art during our discussions. I suppose it all depends on context.
Sometimes it is important to point out issues involving specific directions in art -- and there is something to be said for friendly rivalries, if that makes sense. BUT in general, such criticism is often little more than a petty 'jab' fueled by ego.