Neither of these have really been a huge factor for me. I've found that with brifters constantly under my fingers, shifting takes about as much thought as pedalling. As far as brake response goes, I have never run my FG low at a low enough ratio that I could stop by backpedalling more safely and reliably than with the caliper brakes. Maybe I just don't have the requisite leg finesse, or maybe my brakes are just that awesome, but I would wager a beer that properly set up, decent-quality brakes should always trump backpedalling.

These are the real reasons. I enjoy the variety of riding different drivetrains. Sometimes mashing uphill in a gear that's way too hard and then spinning like mad down the other side is what I feel like doing, just not all the time. I would definately argue that putting in some hours on a fixie can help improve your cycling. Also, reliability.