If I have a library that let me curry trailing arguments for a function, then I can see how something like std::flip() could be useful in letting me curry different arguments without costing extra lines of code. The library I had in mind is Google's RPC callbacks:
I am insufficiently clever to imagine a non-silly situation in which it is useful. If authors only present silly use cases then I am inclined to suspect their creations are only useful in silly cases. If it were useful in solving real problems they should show that as an example!