I think your response here and to the other poster above is quite sound. I also program regularly in Matlab, R, and Python, but when it comes to data analysis, I do find that R is just much more concise than the other two (the data manipulation and statistical analysis tools are more high-level). Learning R and going through its tutorials (the MASS book and actually the S-PLUS Statistics Guides) and learning about functions available to me made me learn a lot more about stats. I sometimes use Matlab for image processing and optimization, or some simple simulation but less and less these days (trying to replace it with SciPy since I use Python a lot in my workflow).

But I also agree that if you're already proficient with Matlab and happy, then maybe you don't need to learn R (though often you can be blissfully ignorant of your possibilities if you are unaware of the vast libraries that another language/environment offers).