I use R almost exclusively and I absolutely love it. But people can get a little carried away with picking and recommending the "best" language for particular task.
If you feel very comfortable and skilled in Matlab, and you aren't finding that there are regular situations where Matlab can't do what you want, I wouldn't really advocate switching. Same goes for someone working primarily in Python.
The biggest reason I would recommend switching away from a language you're already very comfortable in would be the availability of statistical methods that aren't present in your language of choice. Statisticians tend to work in R, so much of the cutting edge work ends up in packages on CRAN.
So I wouldn't dump Matlab if you're happy with it, unless you're just looking to learn something new, out of intellectual curiosity, which is always fun.