perhaps so folks who change their minds 'bout cancelling a PM can do so, provided it's not been deleted yet
or, perhaps, for the very curious who hope and/or persuade the sendee to restore the cancelled PM
or maybe for the packrat-y who can't stand to delete a PM that's been cancelled
that's 'bout all I can figure, anyway
ETA: saves havin' to retype and resend a cancelled PM