The main reason why I have a messenger bag at times is because I can access it's contents without stopping, taking it off, unzipping this and that, rummaging, digging, finding, and drawing out, whatever I need out of it..
'course, when I'm walking 'round with it, it's notebook or sketchbook that I'm after..
Also, there's just something about walking 'round with a big (no really, really really big) backpack that makes me feel awkward. Only people 'round here that use 'em are school children and tourists.
Also, since I'm quite "chesty" the shoulder straps against the armpit area accentuate my chest more than the single strap of a messenger bag does.. even with the binder.