I like singlespeeding my commute, it gives me a chance to rest a bit.
When I worked out that my freehub was going south the other day (on the 27-speed Kaffenback), I essentially cycled the entire way home, so if the freewheeling mechanism seized, I wouldn't be reminded by being thrown off at 20mph! I was knackered by the time I got home, and I still freewheeled to get on and off the bike.
It also means I can use a lowish gear, say 65"-ish. Most people on fixed seem to use something higher so that they can survive going down hill without having to brake too much, or pedal like a nutter. This means that uphills become a bit hard work (although I've seen people on 70"+ get up Ditchling when I walked on 63" !)