Currently, if a repository is deleted, and pull requests that have been issued from that repository upstream are lost. Any comments, code review and so on that were left are no longer accessible, even if the pull request (and thus the changesets that constituted it and that were commented on) has been accepted.
Having the ability to retain these comments puts control back in the upstream repository, where the comments are issued from, where the pull request is listed, etc etc. Even if only the accepted pull requests are allowed to persist, it would be an enormous improvement. Right now, this can be very problematic for projects that do code review.