If you continue to vacuum with a bag that is over half full, you cut the cleaning efficiency of your vacuum by 50 percent or more and increase the strain on the motor. The same is true if you re-use disposable bags because the air flow becomes restricted by the dust particles trapped in the fibers of the bag.
It is a good idea to change the bag at least once a month, half full or not. The dust and dirt you pick up becomes a breeding ground for bacteria and odors. How long do you keep garbage in the house?