perlpod(1) is already installed by default on Debian and
RedHat-based systems; and probably most modern *nixes; pandoc(1)
(and Haskell) are not.
POD also more standardized than Markdown (which flavor? :P),
especially for generating manpages. So save any potential
documentation editors some disk space by not forcing them to install
Haskell and pandoc.
Finally, I'm a mildly proficient in Perl and do not know Haskell
at all and have a better chance at reading/hacking the source if
the document generator breaks.