That is not necessarily so, as the leg extension exercise done wrong (as is the case in 99.9% of the time), carries with it a devastating prospect for knee injury. This occurs once the lower leg travels back (an inch even) beyond a 90 degree angle from the torso.
As you can see from the above image, the knee is already placed slightly back, in a very weak and vulnerable position, placing great shear forces on the knee joint. By extending the knee from that position, the lifter has in turned made a knee rehabilitation exercise become one of injury causing. As for real life practicality (carry over benefits if you like), this is only manifested when it comes to kicking movements in martial arts, as the video below help demonstrates. Fast forward to 0:35 seconds if you can't be bothered watching the whole clip.
I'm saying all the above in the light of having participated in the most dangerous iron sport of them all: Olympic weightlifting. I do not see any danger with the squat movement, but see plenty when that same movement is done in a very dynamic and explosive manner as is the case with weightlifting when the weight is being received in a very low (vulnerable knee) position.