I use a backpack for my commutes. I had to get a rack for the winter commute due to the extra clothes I had to pack back home but I wanted the weight evenly distributed on my back and was always afraid of having the messenger bag fall to one side while riding. I've been commuting for almost 2 yrs now with just a boring ol backpack and it's worked great.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"