I've written up some interesting use cases on this here:
http://dev.zeraweb.com/design-blog-14/

My experience with this is that Matz is doing the right thing. It is
counterintuitive that sending a message to an object should bypass the
guidelines set by the class author. Given that 1.9 is willing to
sacrifice backwards compatibility in order to improve the language, I
don't really see a problem with fixing this semantic mismatch.

Minor sidebar: One thing I think is a little confusing is the version
numbering. Normally, you wouldn't expect a minor release version to
break backwards compatibility. I am not familiar with the roadmap, but
as a Ruby user, I find this a bit confusing. I wonder if people will
upgrade too quickly, not realizing what they are getting into.

This is particularly relevant to send, since I think changing send's
semantics can introduce some very subtle bugs that might not show up
right away. For example, a use of send that invokes a private method
will now invoke method_missing, which in turn might do something that
seems reasonable but is not the intended behavior.

Dan