I'm really surprised to hear that. As I said, I can't really imagine use-cases for auto-punching holes in a file-based cache. Would a web-browser use it for its disk-based cache? I dunno, why bother?
Besides, filesystems would all need to be updated to support volatile files. And the existence of files whose data which might disappear at any point without anyone touching them seems a radical new feature for a filesystem, which seems to me like it could cause all sorts of problems.
But volatile anonymous memory pages? Yes please! That's certainly of use...which is probably why everyone is confused by this!