As a long time reader but only a recent member, I have often found the arguments in comments to be helpful. They help to shape future answers and often the questions themselves and are useful to understand the rationale as to why a person's mind was changed or what it took to convince them an answer was correct. If a user deletes their comments, then you only see one side of the argument from then on which offers impartial information and may not be enough to convince future readers.
I understand when comments are moved to chat or deleted by a moderator for breaking rules; but when there isn't a good reason for the deletion, I believe you should still be able to read what was said.
Should there be a revision history for comments as well? Similar to Wikipedia's talk page or the revision history that already exists for questions and answers.