PyData DC 2016
According to the most recent IEEE language rankings, R is now the 5th most popular language. It is the only domain specific language in the top 5, behind the general-purpose languages C, Java, Python, and C++, respectively. It is common for data science teams to work in multiple languages and for the Pythonista, a working knowledge in R is useful for collaboration, analysis, and performance.
It may be better to extend a simple analysis in R than passing around CSV files. Even worse, there might be a new analysis package that only exists in R, or certain bits infrastructure are already implemented in R and it needs to incorporate a bit of Python analysis.