@jterpslaw Of course you can also use motion sensors to turn lights(and other things) on and off as you move about the house.
From a security perspective, it’s never good to rely solely on one sensor, or even one type of sensor to provide protection. Window sensors can be bypassed by using a strong magnet to trick them into registering the contact as still closed. Much harder to fool motion sensors unless you are willing to attempt Mission Impossible hijinks.
As you are reading through the forums, you will notice lots of different ideas on how to use the ST ecosystem and to personalize and customize it to match your needs. You may start out looking for a security only type of system, but then start adding convenience functions such as lights and other home automation functions.
With that being said I would say this, start by deciding what you want the system to do for you right now and buying the appropriate hardware to accomplish that goal. If you have budgetary constraints, there are always compromises that can be made.
For example I have ST Multis on all of my doors and windows that are regularly opened. I skipped putting them on several windows that are less accessible from the outside and are in the coverage areas of my motion sensors. Instead I spent the money saved on some ST Arduino shields and to automate my garage door.