So where would the shoulder bag fit in? Well, I would not dare to imply that the shoulder bag is going to be a replacement for the standard backpack. But I would say that there are plenty of scenarios where the shoulder bag fits the bill for what you are needing to accomplish. Just think about a satchel of sorts that can fit a canteen and food, a medical kit, NVGs, and binoculars. This is just an example of a few things that you might want to carry on a short patrol, a scouting mission, or hunting. If you are wanting to narrow down the bulk that you are wearing on your person to just ammo and a pistol, this is a good way to store your extra gear in a place that is easy to reach, but still out of the way in a manner that will not impede your range of motion.
In my experience, a backpack can be annoying and make using your rifle challenging, even with few items in it. With a satchel or shoulder bag, I have found that they do not interfere with shooting in any way, and it is for this reason that I began to favor them for carrying just a few items. Such issues may come up when travelling through thick brush in a wooded area. The best thing you can do is maintain a slim and low profile to get through some types of brush you may have to get through. Things like crawling to get under a log or squeezing through close clusters of brush can be challenging when you have a pack on your back that doubles your width. Why go through all that trouble when you can have a shoulder bag or satchel that you can move around at your convenience while you move through these obstacles.
One of the big problems some people would have with a shoulder bag is the issue that it is loose and isnt tight to the body when moving quickly. This is not necessarily the case with things like the Polish Bread bag where it has a short shoulder strap and retention ties. This allows you to keep it close, and even tie it down until you need it. And when you need your gear, this method pays for itself by being very close and all you will need to do is move the bag in front of you and grab whatever you needed. With a backpack, you will find yourself having to take the whole thing off in order to get to its' contents.
This brings me back to the point I was trying to make in the first place. A shoulder bag is not a replacement for a backpack, but it is a good substitute for it when you are not carrying more than a few things that you will probably need to get to quickly over the course of your travels whether that is on patrol, or walking through the brush on a hunting trip. I personally find myself using these things more often than a backpack lately. If you don't have that much to carry, but don't have the space on you to carry it all, consider a shoulder bag or satchel.