I totally agree that Ruby + R makes a great data toolbox. I use Ruby + R for almost all of my day to day exploratory data analysis work.

As far as python goes I think python has become more popular simply because it has more community around data applications. Unfortunately a lot of people view ruby as just Rails. I think academia's adoption of python has also helped it grow into a data analysis language.

Really any MR you do with python you could do with Ruby as it all uses hadoop streaming.