Quote Originally Posted by BDC
It's that it won't be turned off which is the important bit. Imagine if it was turned off for 'safety' just as a few airliners are landing. Could lead to lots of dead people.
It cannot be completely turned off. It just turns on selective availability. It causes the following problems:

degradation of the GPS range signal
orbit errors (ephemeris)
clock errors (dithering)
SA on: position accuracy  100 m.
SA off : accuracy  10 m.