The text in a notebook is written in the simple markdown language, making it a rich format capable of "rendering" latex, links, images and even html (since we're in a browser and the R kernel let's us).
However, the philosophy of markdown is, above all else, to make it readable in either its rendered or plain text format.
Amazing fact: in 2014, there were 80,000 jupyter notebooks on github. In 2015 the number almost tripled to 230,000. This shows how popular and fast-growing the usage is in the community.
More specific stats on the percentages of those notebooks which are using R may not be a representative dataset to glean a trend from at this point given the project is still very young.
Originating in 2001, it was created initially by Perez as an enhanced interactive python shell.
IPython was initially created to fill a need for an enhancement to the Python interpreter. Something useful in addition to simply running snippets of code line by line.
As an aside, it was so good at OS-level actions that some Windows users adopted it in place of the native command prompt.
In December of 2011, Perez, Brian Granger and Min Ragan-Kelley figured out a way to convert the interactive shell into a browser with "cells" where code is executed. Also, there are "cells" specifically for markdown text.
The R kernel (IRkernel) is a much, much newer project with it's first release of 0.1 on Mar 5, 2015 (source: https://github.com/IRkernel/IRkernel/releases)
%%)rpy2 and is currently a common option for combining the IPython features with R in academic pubs"Jupyter is an amazing project that feeds and rides the rising wave of data science."
- quote from article by Alex Perrier here