Apologies, please dismiss that last explanation. I did some more research and have been told that the yahoo archetype was to be used internally so that the dimensions.json and thus, the contexts, are more in line with the Yahoo! Configuration Bundles (YCB) (https://github.com/yahoo/ycb). You can see the pull request for more information as well: https://github.com/yahoo/mojito/pull/694
The yahoo archetype is supposed to be deprecated in any case. I'm still working on updating the docs. Again, sorry for the confusion.