If I leave my webcam (bultin Macbook) always on, may I reduce its life? If I leave my MacBook iSight camera always on, does its life get shortened?
I've worked with hardware for too many years.  My guess is that your camera life will not be significantly affected.  The camera is small with no moving parts, and fairly mature technology.  

If you are leaving the computer unattended, and it is facing towards the sun (maybe sunrise or sunset?) you might want to take steps to make sure direct sunlight will not reach it.