Being Inclusive and regarding opinion
Well. I think you know some of the answers, which doesn't make your questions any less valid. I think people add their cameras and lenses to their signatures so that others don't have to ask. It's mildly interesting in most cases and doesn't bother me. And knowing that someone is a good photographer does lend weight to some kinds of criticism, don't you agree? When, for example, Liquidstone (where is he these days?), aka Romy Ocon, says something about bird photography, I pay especial attention because his photographs of birds are superb. But the bottom line for me is that any thoughtful criticism is appropriate and useful; comments like "that's nonsense" or "you don't know how to use a camera" and any other rude comments aren't.
Now here's my pet peeve, and that's people posting images under the heading of "See? My old you name it still takes great photos." Or "Isn't this lens/camera/software wonderful?" And the photos they post could have been made with any camera/lens/software, or are lousy anyway, or are so small that you can't tell anything interesting about them.
Happy new year.