I use a backpack too, although I have a messenger bag that I use when I need to carry more stuff once in a while. Messengers use messenger bags cuz they need big, waterproof bags cuz they never know exactly what they'll be carrying. Documents, tubes, boxes, etc. For commuters, it's a matter of choice, possibly needing something that looks like a briefcase (look like a cool professional or a nerdy student?), or a fashion choice (look like a cool messenger or a nerdy student?). Just my 2 cents.