A better design approach to achieve this effect is wrapper objects (similar to jQuery and Underscore.js) where I can add functionality to existing objects. Ruby has everything we need for this already, and makes it quite easy to do.
In summary, they makes the Ruby language harder to implement, and Ruby code harder to understand. You can't pretend they don't exist if their existence compromises the meaning of your code and the performance of your chosen Ruby implementation.
Since so many people dislike them, I wonder what the community response will be? Will JRuby just leave them unimplemented? That seems bad, but I'm not sure I'd mind too much. But that's not viable if it means Rails and other things won't work on JRuby.