Configuration is the normal legacy means most Hibernate users build a SessionFactory. Moving forward into 5.0 the way to build a SessionFactory will be much different to allow for people to hook into all phases of building the SessionFactory.
However, we can still allow most users to use Configuration. Configuration in that sense should be viewed as a means to build the SessionFactory in one big shot (aka, "here is all my config info, not build me a SessionFactory"). To that end we would need to remove some problematic methods that allow access to the in-flight state inside the Confirguration.