One reason there's such a plethora of tools a data scientist supposedly should master is a fundamental lack of understanding on the part of companies regarding what a data scientist actually does. I recommend reading "Analysing the analysers" which discusses the different kinds of data scientist and the relevant skillsets. It also touches on the employer ignorance aspect. http://www.oreilly.com/data/free/files/analyzing-the-analyze...

It's pretty obvious from that list ( tools and the number of times they show up) that no, employers do not expect data scientists to know every tools, unless of course there were actually only a few data scientist jobs with such frequent tools as R or python mentioned multiple times in some of them.