I'm also a SS MTB rider (although I also have a geared bike). As others have said, the simplicity of the ride is a huge benefit. It is different, though, than riding a geared bike - momentum is more important, line choice is more important, etc. I ride my SS (it is a SS-specific frame from Soul Cycles) anywhere I would normally ride my MTB.

I also have a SS 'cross bike. This is an awesome ride both on the trails and on the road. I have raced it at the 'cross races, and it isn't a hindrance (except in the final sprint!).

SheFly