Easier to put that weight on a rack then on your back. You might not feel it the first few hours, but you'd wish you never did after that. I'd only put a camelback for hydration. Any other dead weight is just dead weight.

I keep water bottles (2 of them) in the frame mounted bottle holders, and valuables in a handlebar bag. I do carry a small lightweight backpak that compresses into the size of a fist and usually rides compressed in one of the panniers. I use it mostly when I'm off bike, but I have packed some light and easily crushed groceries in there when my panniers have been full, then rode to the next campsite with it. It's ok for short distances, but based on that experience, I'd never carry anything heavy in my back when touring.

--J

I have seen cyclists with a heavy, full sized, fully loaded backpack. Bad idea.
I carry a small minimalist, unpadded day pack but usually it holds my foul weather gear at the top of my rear panniers. I wear it occasionally with lighter, bulkier stuff if needed or for short-range shopping trips from the campsite. Its useful for hikes and doubles up as a pillow.
Some small-wheeled/folding bikes have very low racks which are large enough to carry a medium backpack. This can make a good option. You can fold the bike, put the backpack on your bike and you are highly mobile on foot. With 2-4 panniers+ barbag and a folded bike you are stranded.
I would worry about hygene when carrying a camelback on an extended tour. Do you clean them?

I've carried a small string backpack on an overnight credit card tour. Also, I've used it to carry some items on longer tours. It worked fine, and I have a 'bad back.' Can't see it bothering a healthy person.

Although I haven't ridden very far with it, I have an Osprey Talon 22 which is pretty popular with the "bikepacking" crowd. It doesn't move around, and has an airgap to keep back sweating to a minimum. For those who haven't heard the term bikepacking, it's basical a type of touring/camping done off-road on mountain bikes and most ride without racks. All gear is carried in large seat bags, frame bags (mounted in the center triangle), and handlebar bags, and backpacks. This leaves the bike more maneuverable in rough terrain than rack/pannier setup.

if it hasn't already been mentioned. checked Relevate Designs or Carousel Design Work for frame bags.

Folks may utter about crosswind, but you'd get the same with panniers. Something to look into for bikepackers mentioned above.

Depends on the individual I guess.
I've not ridden my bikes much with a pack. But I've ridden a whole lot of motorcycle with one.
I zip-tied a milk crate to my dual sport's luggage rack so I wouldn't have to wear a pack anymore.
Might look a little silly, but it's a night/day difference.
I even had the pack adjusted so most of the weight of it was on the seat behind me.