I do not see any benefits of having adapters in different namespaces. Remember that Database = ORM so you can have multiple storage without problem just by using proper ORM like DataMapper. 
Having two or more ORM's in the same time is really bad idea and I do not see any scenario where it could provide anything except mess :) and for sure it will be overkill.

Remember that here Database class is just dummy class for providing possibility to test core functionality and give others example of API which should be implemented in new adapter. So you can create special extensions which will provide on top of that  namespace features. But still don't know why someone would like to have it.