It all comes together: it is a matter of debate, however, whether the distinction between resources and their representations is too Platonic for practical use on the web, though it is popular in the RDF community.(Wikipedia on REST)
Backpack actually does reuse controller actions for separate clients. It has a mobile version, an API version, and a regular web version. And about 80% of the actions for all three are shared across through various hacks.( DHH on HTTP-Accept)
And there you have it. Resources are off in the land of Ruby^H^H^H^HForms. Representations are downloaded into the world of Becoming over HTTP.
Too Platonic? Bosh!