Not stupid, a good question.
No, you actually don't need to install CFMX on a separate user.
But it is a good practice -- the same way that the MySQL and PostgreSQL databases require separate users -- to isolate them from other users.
If you recall the Linux install, we installed CFMX as the root user but the install sets ownership and permissions for a named user -- we chose cfmx.
When you start CFMX on Linux the shell script temporarily assumes the id of cfmx, starts the CFMX system, then reverts to the original userid.
This is similar to the way OS X installs software updates while you are logged in as a regular user,
I chose to mimic the way that the Linux install works for the following reasons:
1) It's good practice and reasonably safe
2) It is easier to do the port (no changing ownership and groups).
3) I hope that CFMX will one day be available for OS X -- if/when it is, it will likely be set up this way.
4) it's never too soon to "do it right"