(Or I'm just the only one who still occasionally uses ms/tbl/pic…)
1. Groff is buggy
2. Groff is slow to compile & execute
3. Groff is not BSD licensed
mandoc has none of these problems. If you want a "generic" roff system, groff is available as a package.
According to http://mdocml.bsd.lv/
> groff amounts to over 5 MB of source code, most of which is C++ and all of which is GPL. It runs slowly, produces uncertain output, and varies in operation from system to system. mdocml strives to fix this (respectively small, C, ISC-licensed, fast and regular).
I get it, nobody really seems to use non-man roff anymore. Still, subsetting and reinventing the wheel (partially) seems a odd solution for this.
Because no one uses roff format for anything other than man pages any more. And even then, it's extremely common to use it only as "object code", output from some higher level/cleaner input format (perldoc, pydoc, docbook/asciidoc). I like that they have high level output formats (pdf, html, ascii) and hope that the package becomes an option in Debian at some point.