Ivan Nemytchenko's reading of REST: you never get the resource itself, only one of many possible representations, and those representations can be very different. Developers limiting themselves to one controller per model misunderstand REST. respond_to with multiple formats inside a single controller action is NOT 'different representations' because each wave (HTML, JSON API, RSS feed, PDF report) needs different data subsets, is generated by different tools, and is consumed differently — it deserves its own top-level controller namespace.