Given the active development in LaTeX and ConTeXt, and how LaTeX insulates the writer from typographical details and ConTeXt gives the user extensive control over formatting thus taking useful complementary approaches (see ConTeXt wiki), are there any reasons to use plain TeX?
[Tex/LaTex] Reasons to use plain TeX
plain-tex
Related Solutions
there is a TeX package miniltx:
\input miniltx
\font\myTT=cmr5 at 20pt
\newcommand\stuff{%
Here comes some text $\ldots$ which should
be typeset $\ldots$}
\newenvironment{morestuff}
{\myTT}
{\par}
\stuff
\morestuff
foo bar baz
\endmorestuff
foobar
\bye
With ConTeXt:
\def\blah{blah blah blah blah }
\switchtobodyfont[10pt]
\starttext
\bTABLE
\setupTABLE[offset=1em]
\setupTABLE[column][each][align={middle, lohi}]
\setupTABLE[column][3][align=right, width=.3\textwidth]
\bTABLEhead
\bTR\bTH Type\eTH \bTH Specs\eTH \bTH[align=middle] Uses\eTH\eTR
\eTABLEhead
\bTABLEbody
\bTR\bTD\eTD \bTD Model 1\eTD \bTD \blah\blah \eTD\eTR
\bTR\bTD[nr=3] Spring\eTD \bTD Model 2\eTD \bTD \blah\blah\blah\blah\blah\blah blah blah blah \eTD\eTR
\bTR \bTD Model 3\eTD \bTD \blah\blah \eTD\eTR
\bTR \bTD Model 4\eTD \bTD \blah\blah blah blah blah \eTD\eTR
\eTABLEbody
\eTABLE
\stoptext
which looks like
Best Answer
Without disagreeing with the other answers, let my try to give an answer to your actual question.
I think that the best reason by far to use Plain TeX is that it will compile in 20 years and produce exactly the same output as it does now and as it did 20 years ago. (Okay, Knuth has fixed some extremely minor bugs.)
Consider the situation with LaTeX today. Various versions of packages can give wildly different results. I've written papers with people and in the 15 minutes before the deadline, we had three different looking PDFs, one of which was over the page limit, from the same code because the page breaking decision had been changed by something. And that's with everybody using LaTeX 2e. Try compiling a LaTeX 2.09 file today. What about Lamport's original LaTeX?
The situation with ConTeXt seems to be worse. It is still in heavy, active development.