Sometimes you just have useful information that would interest the original poster, the people who helped him out and/or anyone who reads the ticket afterwards because he has the same problem.
True but leaving comments open means more emails. As one who is actually assigned to a lot of these bugs (and has to act on them), I get 30ish email threads a day (not sure of individual emails - thanks gmail!) from open bugs alone. If we start allowing comments on closed bugs, they're going to send me emails too and the actual stuff I need to do is going to get lost in the sea of emails.
Not allowing comments on closed bugs is about signal vs noise. I, for one, would very much prefer as little noise as possible.
You have the option instead of closing the bug, or denying comments, to just remove yourself from the notfication list or from the assignement. The problem here is that your equalizing noise to you, for noise to everyone else. -- damjan