Looking at an ACS classic 3-3-1 install (don't have a version of openacs or ACS2.3.3 handy, sorry), I don't see anything that sets user_demographics. There is a query in admin-procs that will use it if it exists, some code in admin/users for updating it or using it, but nothing I see sets it.
find . -name "*.tcl" | xargs grep -i "demographics"
is probably the best place to start. I don't know the history of this table so it may have been used to a different extent in earlier versions of the ACS.
I would think you would want to modify your registration process with optional fields for entering demographics info (or a new page for entering info, depending on how much you want to put up front).
