This seems like an absolute requirement.
It was asked before in the forums, and the response was that it would cause problems if a task was sent to another user, they followed the comment, and the original task creator deleted the comment. This is a poor excuse for not having this functionality. If this is a problem, then just have a deleted comment flag, with a "Show Deleted Comments" button.
There are too many reasons why users need to be able to delete or edit a comment, most of them are common sense, so I don't think I need to provide rationale for the need. Users requested and need this feature.
I have to say thanks for the great product. It exceeds others I have tried, and has great features I didn't know I needed, such as the Do It Now lightning flag. Keep up the great work! (I will be buying an annual membership for sure, just can't recommend this product for my organization until the delete/edit comment feature is added. It is too much work to have to delete a task and create a new one, just to delete a comment.)
Interesting, the first thing I noticed after posting, was the big huge red "Delete" button on the bottom of my post. Reason enough to add the feature to task's comments? :-)