Not sure, will investigate more. However, looking at the test case for ServletContainer, it looked that the method was used quite a bit as well... so I figured it should be on the interface instead of the default implementation.
I think I need a better understanding of the web module to give a definitive answer.

Considering that ServletContainer is supposed to be a static registry for ServletContainerContexts (at least according to the javadoc), it seems natural to me that you should be able to register contexts with it. Also, the ServletContainerContext interface makes several references to such a registration (cf. Registration inner interface for example).
The register method is used by JBossWebContext in Portal.
With the addition of add/removeWebAppListener on ServletContainer, the whole Registration concept seems rather convoluted and seems to make the API more complex than necessary (though I might not have a proper understanding of the big picture).