Comments are considered expendable on every SE/SO site.
Comments are disposable: unlike posts, there's no revision history, and they can be deleted without warning by their authors, by moderators, and in response to flags.
Flagging is an important form of community self-moderation, a highly regarded privilege on these sites. There are even badges awarded for helpful flags (often raised on comments.) Handling flags is one of the most important duties of moderators; it's one way to allow the community to participate in moderating and shaping their site.
If you make a comment in response to someone else (in effect having a conversation with that person, discouraged on SO sites), and the person you answered removes their comment, your comment immediately becomes obsolete, and if flagged by a user, the mod usually will delete the comment. This is true on every site I'm on. It's a very easy decision, since comments are not meant to be answers, are not equivalent to answers, and usually have no lasting value, especially if the comment was part of a conversation half of which is now missing. All it does is clutter the page, possibly confuse readers, and distract from the actual question or answer.
Could moderators please delete comments with less precipitation? Or even better, quit deleting comments that are relevant to the question. Thanks.
I hope this is never the case, as it goes against the spirit of SO. If you want your words immortalized, write an answer. That's the model.