In today's world, saying truth which might hurt someone is worse thing you can do. So nowadays being nice (and not honest) is somehow must, and everyone have feeling, they need to compliment someone just that he or she might not be offended. On top of that, A LOT of people take it personal and feel offended when someone finally tells them truth and tells them their photo(s) suck. That's main reason, why I don't bother anymore commenting. Because when I did comment, I was honest, and I was actually looking for flaws. Not because I would want to offend someone, but because I think pointing out mistakes is way to learn something new. And in my mind, if you put photo out there for comment, you should be able to accept critique which might be different then just a praise.
But a lot, if not most, of people are not ready for that. So instead of having few 100 people as enemies (I have too many because of my non-critique related comments already ), I rather quit commenting. I am who I am, and it's just not me, to tell "whoa great photo" to every crap I see.