[urbanautomaton]

It doesn't seem to be widely acknowledged amongst people promoting this particular style of DCI as a good way to write Rails app, so I'd say the point is valid, even if the title is overstated. There are also quite a few other very popular gems that make heavy use of #extend at runtime for non-DCI purposes, including Haml and Paperclip, so I don't think it's fair to say that this is common knowledge (at least in the Ruby community).