I quite often want to email diffs of files I don't have write access to, so I visit the file, write it to /tmp, then edit it, save it, and go to /tmp to run diff between the file and its backup. But Emacs doesn't make backups of files in /tmp, and there doesn't seem to be any way of asking it to via the customize interface.
The best I've found is to: (setq backup-enable-predicate '(lambda (file) t))
Suggestions: 1) make that the default value - why not write backups in /tmp? 2) offer a way for the user to customize the behavior without resorting to writing Emacs LISP code