Leg extensions are used quite a bit in rehab.Some based on the Nautilus cam, started by Art Jones. Also bands, cables and even hands on resistance are used. They have other muscle rehab machines that some BB'ers never dreamed of, at injury recovery clinics Thing is you never want to extend the leg so high that the knee has stress on it. The point is to work the ligaments, tendon and muscles around the knee, but not the knee joint it's self.
The same goes for regular leg training for BB'ing. Never really want to lockout at the top position. Leg extensions are excellent for pre-exhaust movements, but never a stand along exercise. If leg extensions are the only quad exercise you care to do, than make sure to do an equal amount of ham/leg biceps work for strength and muscle balance. Opposite muscle groups should be trained equally, to avoid potential injury. Good Luck.