I'm not a game developer expert by any means but i've seen many big game dev compaines handle stable and beta branches in a better way so here are my thoughts:
As you said, OB should be used for testing new features. The problem is that they also use it for pushing the bug fixes that stable has so then you have to wait a long time for ED to push the big patch that contains a clusterf*** of things: FIXES/NEW FEATURES/UNRESOLVED NEW BUGS etc. Doing that will never result in stable being stable. To prevent this i think they should test the fixes internally and push it both ways, stable and beta.
Then, in beta, they should submit a bunch of new features and stop there (not bring new ones) until they fix all the bugs that the current ones brought and then push them on to stable.
Hope this idea helps.