[mrlmd1]

Most of the packs specifically made for bikers are well padded where they contact the rider, are made out of pretty tough material, and actually may and probably do provide some additional back protection during a fall or slide on the road surface. Nobody in any site I have seen has ever complained of small items poking through or protruding on impact or even spewing all over the road. They carry the load pretty low and are not that obtrusive to riding around at moderate speeds.
If you have to carry the contents around with you they are a viable option, although I would have to agree that saddlebags may be better in some instances but they provide very little if any security to items left on the bike, then you still have to pack them up into something else to carry them. If a backpack s not securely attached to the bike, it will be less secure (more prone to breaking loose) and not everyone has a sissy bar or something else to attach the pack to. Like everything else on here generating heated discussion, a good backpack is just another viable option, and if you are going that route, get one designed for a biker and make sure it fits, etc, etc.