If you had a 2 speed crank with the S3X then you would get 6 speeds. But, one might get a pretty awkward gearing range. So, there may not be a big benefit.

I have wondered about modernizing the old Cambio Corsa by adding handlebar controls. It should be capable of running fixed gear. I don't know how many teeth would be practical, maybe 9,10,11,12,13,14 (5T) which could be used to give a pretty good gearing range, especially if coupled with a front derailleur also shifting 5T or so.

As far as usefulness. As Homebrew01 mentioned, I also like to coast a bit. Part of the idea of the fixie is to break bad habits (and maybe make new bad habits).

A direct chainline might be slightly more efficient than going through a derailleur, but one might loose some of it if shifting is cumbersome.