Joined: Aug 2008
Posts: 13,140
Likes: 2,162
From: Ann Arbor, MI
Bikes: 15 Kinesis Racelight 4S, 76 Motebecane Gran Jubilée, 17 Dedacciai Gladiatore2, 12 Breezer Venturi, 09 Dahon Mariner, 12 Mercier Nano, 95 DeKerf Team SL, 19 Tern Rally, 21 Breezer Doppler Cafe+, 19 T-Lab X3, 91 Serotta CII, 23 3T Strada
There is no compelling reason to ride with a backpack.
I understand it may be what you have, or you can't afford anything else, but those are reasons not to buy a messenger bag or rack/pannier setup, not good reasons to wear a backpack.
Honestly, unless your off-bike activities require a backpack, get a messenger style bag at least. It will solve your problems and enhance your riding experience.