This really does signal the end of the thread, I mean why would you carry weight on your back while cycling? What's the point? As others point out it's bad for aero, bad for balance and stability, bad for your back, bad for comfort, there really is no argument that can overcome those obvious truths.
The answer, as everyone with a brain knows, is to carry the bag on the bike, as low down as possible, preferably at the front or rear, if you can't fit everything you need to carry in your bike then get side panniers, if you have side panniers front and back, plus a toptube frame bag, a handlebar duffel bag, a seatpost tail bag, rack bags front and rear and you still need to carry more stuff get a trailer, and if your trailer is full then OK, sure, in that extreme and improbable situation then go for it, get a backpack, but that is the ONLY time such a foolish baggage choice should be made.
A backpack should be the absolute last resort when all other methods of stowage have been exhausted.