I know that the official policy of Joel/SE is that comments are ephemeral and subject to unlimited deletion at a mere wish of a moderator.
However, I feel that deleting comments - which leaves ZERO backup of the comment data behind - is a very unthoughtful and ungratious thing to do to the users.
My problem is not with the deletion per se, but with the fact that the commenter(s) may have - as I have been known to do - spent HOURS researching material and polishing wording to put into comments. Wiping out that effort with zero option of recovery is basically a sound slap in the face of a person having spent the time and effort.
If possible I would request that the moderators adopt one of the two approaches, when deleting a comment thread that is not 100% clearly filled with junk only:
Somehow back up the data in the comments (by forcing into a new chat room? I'm not familiar enough with SE tech to know if there's a good solution).
This need not be permanent - again, only a temporary backup to make sure people who put in the effort in compiling research and wording can recover it, to post as an asnwer/edit/question/blog post/whatnot.
Warn the commenters that the thread is about to be deleted, allowing them to back up the data.
I realize that some comment threads are in possible need of an ASAP cleaning (e.g. when they degenerate into offensive language/name calling etc...). But a vast majority of threads that are deleted are merely "too discussiony", and not real-time-offensive, therefore allowing a grace period before deletion shouldn't be problematic.