Having spent last 4 months interviewing at several companies for ML roles, I can assure you that no one doing PhD in Computer Science (Machine Learning, Vision, Data Mining) uses R.

I also dont understand what you mean by "exceedingly high value work". For both production as well as research (ICML/NIPS/CVPR) the languages used are in most cases Python/C++/Lua.

Also Stats PhD (who I believe are the sole users of R) aren't typically hired into Machine Learning roles.

R is okay for wrangling tabular data, applying statical model for hypothesis testing and generating pretty charts for papers. But not suitable for state of art Audio, NLP, Vision or Reinforcement Learning.