My main camera bag is a backpack style bag...also, I know & work with a few wedding photographers who use backpacks as well.
The think is, that we never really shoot with them on. They are just for transporting gear from one place to another. I'd actually like to get a large shoulder (or maybe wheeled) bag to use for weddings. My main problem with my backpack style bag is that to open it up, you have to have the straps on the bottom...so if you are in say, a wet/dirty field, you have little choice but to set the bag down (straps on bottom) onto the ground. This could obviously make you dirty when you put the bag back on.
With a shoulder bag, on the other hand, the bottom is always the bottom. You could set it into a mud puddle and still access it and then pick it up and go.