normal backpacks with the larger straps can dig into your neck and under your arms from the stretched-out position youre in while riding. i cut mine down since i dont feel like getting a messenger bag. when the messenger bag strap runs across your chest...it has no place to dig in and can be more comfortable. i still use my normal backpack and im happy with it.

edit: im not a messenger. i just carry my books to work then class and back. and even with a few books that thing gets full pretty fast.