Personally I don't comment on shots that I think are rubbish. So then if the shots are already good to start with (and of course I mean 'properly good'), it is always going to come down to a matter of opinion - or even just thoughts and ideas. I've had situations where I made a suggestion, than afterwards downloaded the image and try it myself, and it just doesn't work anyway.
Then, yes on average maybe only 1 in 10 comments (or less) will make you think - hmmm, that might be an idea. But when you are already experienced and tried quite a number of things, it is hard to get good/constructive ideas.
So, I just treat people the way I would like to be treated - I'd rather someone said something because I can always ignore it (or just agree, but maybe it was something I had no control over), but just occasionally you will get a gem, and those are more valuable than I can get from anywhere else. So that is the main reason I will make a comment rather than just do the easier thing and be quiet.
Plus, for instance I might move a rock or use liquify to bend a tree or change a color or something else that other people may not do for their own ethical reasons.