Personally I found backpacks to be terribly uncomfortable and leading to backaches and other issues. Generally they're designed for a very upright posture and when used on a bike they exert much more pressure on my upper back than when I'm walking.
If you and your back are happy with your current backpack you may as well use it - but it didn't work for me. Of your listed pros for backpacks, the only one that holds true for me is the ease of carrying while off the bike. The other items seem just as applicable to my panniers.