you may have to shift them around a bit so that you don't get a bolt digging in to your ass, and it probably wouldn't be very kind on your shoulders to carry them for miles and miles. They are also quite difficult to maneuver indoors, and subsequently I could imagine them being hard work to carry through busy streets (I do most of my bocking in the countryside, so it's not an issue for me).
The straps of the bag don't seem to be under too much strain, but I think it would depend on the build quality of the bag as to whether or not they're likely to break.
As for carrying the backpack, I think it's easier the more stuff you have in it (I only had a bottle of water, and could feel it bouncing up and down with every step). If you were to fill it up, or secure the things inside it it would be easier. Also, having the backpack on the tightest setting stops the bag itself jumping up and down on your back.
