I am not so sure about the scope of the 'bleeding edge' part. It is popular among old school, statisticians. Another population (no pun intended) that R is popular in is one where one knows neither statistics nor machine learning but just wants to try out a laundry list of canned methods without a need to understand: pretty plot goes up, yay awesome; pretty plot goes down, ok try the next algorithm (or the other way round). Here I think R is pretty unbeatable in its breadth.
It is not that popular among machine learners. Part of it is cultural, a typical machine learning person will be coming from a CS background and then R grates more.
It has been claimed that R is lisp meets stats, I think that would be Julia now and Lush then. http://lush.sourceforge.net/. To quote Yann LeCun:
"Lush combines three languages in one: a very simple to use, loosely-typed interpreted language, a strongly-typed compiled language with the same syntax, and the C language, which can be freely mixed with the other languages within a single source file, and even within a single function."