I've had to do that because of the massive number of people who post solely to get increased post counts (basically spamming every single thread they can find to try and rack up thousands of posts a day). This helps keep things more clean, and I actually suggest that anyone who creates a thread do it moderated.
I'd never delete a post that was on-topic, whether it was positive or negative. I just get tired of having to filter through people posting things that have nothing to do with the topic.
For what it's worth, I've never had to remove a post here. I just learned the hard way from other threads, :p.