General discussion

To prevent manipulating. For example, imagine you are the owner of a malicious site. So you post a bad comment and wait until, say, 10 people have agreed. Then you edit your comment so it turns into a good one, and suddenly 10 people who think it's a bad site automatically agree with you! It doesn't even need to change the category - think of "Other", for example.

Basically, it's that people who agreed with your old comment might not agree on your new one, you see.