A rear brake on a fixie can be useful when touring. When you're bike is really loaded down and heavy and you're flying down a hill your feet and a front brake will still stop you, but it's a bit of effort. With a rear wheel that would be less effort. Also, when I toured on my steamroller I felt like I wasn't letting myself go as fast on the downhills because I wanted to stay in control, if I had a rear brake I probably would have done some of the descents much faster. On an unloaded bike I don't think a rear brake really adds much stopping power, because under normal circumstances you can provide more than enough rear braking power.