It mostly depends on what you want to do. If you want to try to beat the speed of the internet like some of the HFT stories you hear, you're going to want to cut as much fat out as you can. If you are more interested in doing some analysis of historic data where you don't care about speed so much, it's a lot easier to play around with things in a language like R or Python.
I hear that R/Python's role in the project is initial modelling and testing. You train trading algorithms in R or Python, then deploy them in a C/C++ algorithm for speed.
