Well, I do have a front brake on my bike already. I kinda just wanted something that didn't look as obtrusive but was reliable, especially in the winter. Also, I just like to tinker with bikes. It's fun in and of itself.
The other thing is that although my fixed gear bike can go faster than my MTB SS, I find that using a coasting bike is just easier when I'm riding around a congested campus or urban enviornment. Surprisingly I've found my fixed gear bike to be a better distance road bike rather than an urban bike. It almost makes me wonder whether a freewheeling bike would be better for messengers or not, right? I mean, just thinking ahead to winter, I can already see my poor braking on the fixed gear as opposed to the coaster brake wheel (the lesser of two evils I suppose).