I believe that load security on the rear rack is less dependent on what you use to secure the load with, but on the configuration of your load, and the relationship of the racktop load with the rack and panniers.
Trying to secure a sleeping bag, Thermarest (or some type of sleeping pad), tent, and poles to the rear rack when they are all separate, is a challenge however you secure the items to the rack. The same items contained in a dry bag or rack pack are relatively easy to secure, and are protected from the elements.
I also think if the rackpack sits on the panniers and the rack it is a pretty solid platform, and there is not much room for anything to shift. Placing the rackpack across the rack so that the panniers stabilize it is good way to go.
If you look through the "Loaded Touring Bicycles" site you will see every configuration imaginable, Some are neatly packed, and some bikes look like they just came from a yard sale. It is not hard to guess which loads are the most stable, stay dry in the rain, and are easier to manage at train stations, and airports.
http://www.pbase.com/canyonlands/fullyloaded