Well, speaking for myself, here....

The attraction for me starts with the premise that with a "vintage" bike, I like to find the best compromise between the original style of the bike and whatever utilitarian purpose I will put it to. So while a touring bike from the 70's or 80's looks best with a Jim Blackburn rack over the back wheel, an English lightweight from the 40's looks best with a saddle bag. The saddle bag on my Fothergill, as shown, has the right look. The way it attaches, and rests on the Park bag support, is not period correct by any means; but that's not the point. The top of the bag is reenforced with two pieces of 1/2" x 1/8" aluminum bar stock. A cable clamp (a U-shaped steel rod, threaded at each end) goes around each saddle rail and through the two aluminum bars. So the bag is very securely mounted to the saddle rails; it does not flop around at all.