I use a road bike sometimes set up with a coaster brake and front rim brake as my main commuter. I agree that adding a front brake to your bike would allow you to brake more effectively, though they are not nearly as necessary as some posters have made it seem. Thousands of urban fixed gear riders can ride effectively in traffic at high speeds with only a rear wheel brake; all it takes is practice and a reckless nature.
Don't worry about riding hills with a decent quality, well-lubricated coaster brake. It takes a mountain descent where you are constantly applying the brakes for a really long time to start boiling off the lubricant. Modern greases will hold up even after the hub body has started to discolor from the heat. Anyway, you can avoid this heat build up if you do a series of skids to scrub speed instead.
Oh, and losing your chain does mean you lose all braking. Pressing your shoe tread against the rear tire works very well in slowing or skidding the rear wheel. Check out Ted Shred on YouTube. The guy's been commuting in SF on those steep hills for years with no brakes except his feet.