Most of my panniers have D-rings for a shoulder strap. The exception is the Bikase, but with that I can clip one backpack strap to the opposite-side bottom clip, and it becomes a sling (akin to a camera sling bag).

But once again, it's muscle memory. If you've grown up riding with weight on your back, it's second nature. If you've ridden your whole adult life with the weight behind you and down low, that's what's second nature and having weight on your back is not.

I still don't "get" using a messenger, however, as that's weight that can shift too easily.