What is a key difference between JSF request scope and ADF backing bean scope?
Well, the "lifetime" of these two scopes is exactly the same. However if you need to create a backing bean for an ADF declarative component, the you will need to use backing bean scope for that managed bean. This enables using multiple instances of your declarative component in a single ADF page...something which the JSF request scope does NOT support.
(Please see the comments on this blog entry for an additional function of Backing Bean Scope as relates to regions and bounded task flows; many thanks to Chris Muir for pointing these things out.)