automated/transparent backup of modified files considered useful

I speak of course of the feature that is very familiar in vi(m) and (x)emacs, where a modified file is automatically and transparently backed up by the editor. The backup is dumped upon manual save. If a crash occurs (<rethoric_irony>even if software is nowadays crash proof</rethoric_irony>, hardware is prone to failure – power, wire tripping, drunk colleague etc.), the ghost backup stays behind and can be detected and offered for recovery by the editor.

This feature is even more useful today with editor features like refactoring (which can affect a great number of lines/files) and mass editing (with regular expressions or with rectangular selections, for example).