I bought a single-speed road bike last year because I didn't want to ship my good bike to the States. As city commuter, in flat places like London or New York at least, it's ideal and has become my winter bike of choice.
I find I work harder on it than a geared bike Do Gears Cause Laziness? and it probably kept me fitter but for longer distances I prefer a geared bike.
How this translates to the mtb road I don't know….