Researching housing does not add to your PR, if makes it the same.
with 7 housing, you get the optimum output of food. Instead of building 10 housing for 100 land, each housing provides 17 now, means you build almost half that of the original. Adding max infra in agri, this gives you quite a significant boost in overall income. The maths has been done.
Hence to say "researching housing adds unnecessarily to your base PR" is untrue, unless the person in question is incompetent and decides not to adjust housing accordingly.
However, it is true that if you do not intend to go long term with your maru empire, then go pure agri; the 7 housing route is only for those who intend to stick with their empire for the long term.