Some real nastiness is possible when a comment can be removed without any trace discernible to the ordinary viewer. That nastiness wouldn't even necessarily be deliberate. It comes from erasing the context within which subsequent comments were made, typically by others but perhaps even by the person who deleted one of his or her own comments.
I understand why someone with perfectly innocent motives would want to delete a comment, and how in some particular context the erasure would not produce misleading results. But there is no practical way to code so that comments can be deleted in most such cases and only in such cases.
(I would, however, suggest that, when two unedited comments each perfectly duplicate the other, the system automatically purge the second.)