I've used it before to provide net access to two machines when there's only been one wire, and neither machine was able to run in AP mode. It was really, really awkward....

I think I used it once to connect a pair of PSPs to play a two player game....

I think the use-cases for true Ad-Hoc mode are actually pretty limited and in most cases one system (android or iOS these days) can probably run in AP mode anyway.