Fair, except that DI in Ruby is pretty dang simple.

    def some_method(dependency = SomeClass.new)
      dependency.do_something
    end

I'd argue that injecting the dependency gives you even more expressiveness and improves readability since you now can name the dependency whatever you want (assuming you give your variables meaningful names and think it's a good thing).

Sandi Metz really does the topic justice in her talks on SOLID design: http://www.confreaks.com/videos/240-goruco2009-solid-object-...