I really think it depends on what you're writing about. For example, if I'm writing about something controversial or with varying view points, I'd welcome criticism. But because I write mostly humor with no debate involved, a criticism would serve no real purpose but to insult. So for me, it was a tough question to answer.
I was running a giveaway and one of the Rafflecopter entry options was to leave a comment on a post. Out of all the posts this woman could have commented on - she decided to pick a recipe and told me it looked disgusting. I was like 'WTF? Seriously?' I wouldn't have the nerve to do that, by all means don't suck up to me, but no brownie points for being insulting! None whatsoever! I despair sometimes >.