Edit: In fact, you can use motion to record images of the motion that it detects. You can then plug those images into OpenCV for facial recognition. Both Motion and OpenCV are very powerful pieces of software and they can compliment each other nicely.
Edit: ... or Reply.
Interestingly, the CPU usage went from 0 to ~30% when I added the first camera, but there was no noticeable increase in CPU usage adding the other 2 cameras.
I use it to record any detected motion, works a charm. It even has a feature to dynamically ignore parts of the screen. Say part of your camera's field of view catches a plant that blows in the breeze, the system will learn to ignore that area of the screen.