When a question is migrated to another site sometimes the comments no longer make sense. While you can delete the comments on the question in the new forum, you aren't able to do so on the original. Making new comments on the question is obviously not appropriate, but there are times when cleaning up comments (much like the answers are deleted on the original) does make sense. Automatically deleting them is probably not appropriate but once the question has been migrated, I should still be able to manually delete comments that now are out of context or serve no purpose.
This isn't a huge priority for me, but if it's an easy fix -- like, just don't mark the comment as not being able to be deleted -- then I think it makes sense.
I've just hit this on this Super User question, so I was going to post a question when it suggested this one as a duplicate. So, I thought I'd edit this one to bump it onto the front page again.