More important than any of the responses so far, by wearing a rucksack you are a) raising the distance between your head and the floor if you come off and land on your back, real bad for the neck in an accident and b) anything hard in your pack could penetrate the body on impact or force the back to bend back in an unnatural manner.
In the UK many medics will tell you not to wear a rucksack for these reasons. I was also told not to whilst on a First Biker on Scene medical course (tells you what to do in the event of being the first person to arrive at the scene of a rider down; when to and how to remove helmets etc - highly recommended)
PS I use Kriega back packs and/or tail packs and/or tank bags and/or bumbags when having to take stuff with me.
Good points to make. Rucksacks also don't go well with aero humps in leathers...