Comments are, by intent, disposable. They are not designed to be part of the permanent record of the post. If something should be part of the permanent record, that something should be accomplished by editing the post.
In general, any comment that is chatty, or outdated, should be flagged.
If you feel the comment has served its purpose, or never had a purpose, then flag it as no-longer-needed. Obviously, the moderator who fields the flag will have the final say.
Having said that, I ruthlessly delete comments that are flagged, and seldom even look in to the context of the comment. If someone flags a comment and I don't see any reason to keep the comment, I will just delete it, and move on. Only if there is something "different" about the comment will I even look to see the comment's context. So, if you want a lot of comments deleted from a single post, you should flag all the comments to be deleted, not just one....
Since this is about flagging comments on the SO site (and not here.... on Code Review), I'll reach out to some of their mods and get their opinion on this as well.