What you know you have to carry every day might well fit into a 12L bag, but what if you want to hit the grocery store or library or buy a six pack on your way home? I have a Banjo Brothers waterproof backpack, the top of which can be rolled up or down to accommodate different loads, and this feature is invaluable. (I was interested in the Chrome bags for the same reason, but they don't fit small-framed people--or at least this small-framed person--that well.) The Banjo Brother bags have no interior compartments, which actually works well for me.