I like that groff is a typesetting system, it means that I can generate pretty postscript versions of manpages from the command-line and print them off, all in one go. It's really handy that way.
You can do that with Mandoc too, although the quality of the PDF output is not as nice. What makes Mandoc less of a typesetting system than Groff is that it supports fewer macro sets (mdoc, man, eqn, and tbl, the macro sets commonly used in manpages); Groff supports more (ms, me, mom…) because it’s a more general typesetting system.
[1] http://undeadly.org/cgi?action=article&sid=20110314142734