One advantage of certain types of 'scaffolding' is that it allows instant removal of the saddlebag from the bike. This is useful on tour when you want to bring stuff in to your tent/B&B/hotel rather than leave it to get slugged, rained, stolen, messed about with by drunks or whatever might happen (but probably won't) or obviously if you actually want the stuff inside it, as well as on longer audaxes when you want to layer up, redistribute snacks, or whatever, in the warm, dry control rather than standing outside in the rain. I don't actually have either of these supports (the Bagman QR and the Carradice Classic rack – others might be available) so I can't compare the speed of removal to taking a rack pack off a rack, but expect it's quicker.