I can imagine one of the reasons it might be useful is if you have a badly written argument parser that only accepts single-dash commands, but you need to pass

I can imagine one of the reasons it might be useful is if you have a badly written argument parser that only accepts single-dash commands, but you need to pass a long argument (that gets shelled out). However, it's entirely opt-in so I imagine this is not practically useful for most programs. Notably, GCC's -W<foo> works the same way. You could do "gcc --all" if you wanted to.