Why ride a single-speed bike?
I ride a single-speed (as opposed to fixed gear) because I like to be able to coast down a hill without worrying about spinning out, or hitting a pot-hole while frantically trying to keep up with my pedals. Don't get me wrong, I love riding fixed-gear, but for where I live it's just a little impractical to not be able to coast.

I ride a single-speed (as opposed to a geared bike) for multiple reasons.

A: It's all I could afford. If you only have $600, you'll get a lot more bike with a single-speed than a geared bike.  Or rather, you'll get a lighter bike with better hubs/wheels/etc than for a $600 geared bike. Someday I'll have the money for a decent road bike (105/Rival or better) and will buy one. But for now, my single-speed is much lighter and sturdier than any $600 geared road bike I could have gotten.

B: I enjoy the simplicity. Want to go faster? Pedal faster.

C: I enjoy the workout. I live in Utah, where there are some pretty big hills/mountains. The only way to get to the top is to (as my dad used to say) hunker down and gut it out. There's something quite invigorating about knowing you conquered that hill with the power in your legs instead of the mechanical advantage of your gear ratio.

D: Even on the freewheel side of my hub, my single-speed is quiet. No gears means that unless I'm coasting, my bike is nearly silent.