Well they might not be hired to do just web development but I think learning about the web stack is incredibly useful because it helps you get your work out to people. This is important especially in a (early stage) start up where you might have to rapidly prototype solutions and you can't have your own pet designer to whip up visualizations for you. In fact, if you read the blogs of some of the data scientists at twitter/linkedin, its immediately clear that they have some know-how of the web stack in the way they are organizing and visualizing results. (Everyone likes pretty pictures)
Not saying you need to be a web developer to write a blog. All, I am saying is that the web provides a way to do interactive visualizations in a manner not provided by Matlab/R/matplotlib. For example, colleague at Mozilla did this: http://skedasis.com/d3/smoothers/
