Personly I think it is a tool for the job. I have lots of bags for diffrentt jobs and diffrent bikes. I like a backpack on a BMX as it moves around less and you are upright when cycling, and panniers of good when going on very long rides but horrible for weaving in and out of traffic. I like a messenger bag over a backpack around town because I feel my messenger bag (well it's not a proper messenger bag but a large shoulder bag which I have added lotts of straps and paddng to) is a bit cooler but mainly both my backpacks obscure my vision when I look over my shoulder but I can posision a messenger bag so that I have a clean line of sight when changing lanes. Also for some reason I find my backpacks pull more on my shoulders when cycling I seem to be able to possion my messenger bag better on my back. So in general I think FOR ME it boils down to flexability on possitions and use. This may be though because my backpacks are standard and great for walking around but I have customised my messenger bag to make it comfy and secure for biking.