I realized that nobody has noted two of the biggest reasons to allow each user to delete their own comments. If you comment on a bug and then realize you made a spelling or statistical error, you should have a way to fix that mistake. You could delete that comment and create a new one with the error fixed. (You could also edit the comment, but there is no easy way to do that yet. See Bug 540 Comment 208) Some comments are misguided or provide false info entirely. Anyone who adds a comment like that should be able to remove the comment entirely on their own.
Bug 282657 Comment 19 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Comment editing is coming soon (Bug 540 is almost fixed), but the ability for users to delete comments is still desirable if it can be properly added. I am trying to get this incorporated into b.m.o (Bug 1396329), in which case it could be subsequently included in Bugzilla.