You don't have to use a rack, that's just what I use. It was more a post to warn you that the large saddlebags require more than just slap it on and go, it's not terribly well conveyed on the carradice website. They are solid setups though with whatever choice of support you go for.
I go for a rack because it's cheap and the load on my commuter bike can be heavyish. In that photo I had, bike tools (quite extensive, I'm paranoid!), u-lock, pump, shoes, work top, packed lunch, biscuits, gillet, arwarmers, map, phone, wallet, engineers square, rule, selection of files, selection of flat drill bits, all heavy dense stuff. Basically far more weight than you would have for a bivi trip, that's why I like the rack. A few good pics in this thread using the sqr system
http://www.singletrackworld.com/forum/topic/large-saddlebags-any-good