I plan to use reStructuredText to write documentation with the main purpose of generating some nice HTML pages. For this I use the docutils rst2html.py tool.
However
I have also seen this done by rendering to html using rst2html, then converting the html to plain text by using a command-line html browser, such as:
Each of these browsers has a command-line switch or similar to render its output to a .txt file, so you could create a two line script called 'rst2txt', something like:
rst2html docs.rst docs.html
lynx -dump docs.html > docs.txt