Bikepacking bags totally puzzled me when they first came out. But then, I don't ride off-road, and I now perceive an advantage in a narrower profile to better clear brush and tall grass off-road, and perhaps better aero onroad.

When I fold my bike for transport or train, I need to take off all bags; 4 panniers and 1 trunk bag are already more than I want to carry. I snap pairs of panniers together, with line around each pair in a cross-parcel pattern if carrying far or handing off to anyone. With bikepacking bags, I'd need one of those big mesh duffles that I've seen others here use, including to consolidate panniers for air travel.