Q:

Is it okay for a moderator to delete all comments just because he was going to delete his own comment? 

In the comment section to this question post, I was replying to a comment of a moderator with a question and also posting another comment for the asker.  The moderator apparently didn't like my question to him and wrote something like "how pedantic you are" (to which I responded that I don't mind if he doesn't mean what he says). 
A few hours later all comments, including some comments of the asker and my comment to the asker, which were not related to the little conversation between the moderator and me, have been deleted. 
I think that if he was going to retract his comments, he shouldn't delete the entire comments section.  That's destroying content.  Isn't he exceeding his competencies with this action? 

A:

To be clear - no Programmers moderator ever made a comment on that question. 
There were plenty of comments on that question.  I'm not sure if it was flagged by a user or an automatic flag raised by the number of comments, but it looks like it was brought to the attention of a moderator who cleaned out the comments.  I think cleaning out the comments was the appropriate action, given what comments are for - asking for clarification or pointing out issues with a post, not for carrying on an extended conversation.  If you want to carry on an extended conversation, that's what chat is for.