There are times I read old mailing list messages where some development was discussed and they mention something in some particular commit. If the history has been rewritten (or a branch was deleted) I can't get the full picture.
For this reason I'm personally very much against the idea of modifying or deleting anything from a repository.
I know me saying this will do little to change anyone's mind, but I felt the need to justify the position of someone like me who sees the repository as a very strong record of the history.
Regards,
Elias