Well, single speed and fixed gear are two different things, to begin with. Yes, fixed gear is a single speed, but you can't coast and don't see many mountain bikes set up this way.

why? Well, for me the fixed gear is a different challenge. You have to power up climbs and spin like crazy going down the other side. It's also incredibly simple riding. You pedal. That's it. Pedalling. No gears, not even a computer on mine. When you're not pedalling so hard you can feel your lungs in your ears, you can enjoy the scenery and not think about riding your bike as much. Of course, you have to remember to pedal, but you see where I'm coming from.