i think they have their place...they aren't the type of movement to base your workout around but can be effective for what they are
some ppl do them at the beginning of the exercise to pre exhaust....i like doing my big movements first and using the extension almost as a finisher...it's a nice movement to pause at the top for the squeeze and get the big pump and stretch to really top off the leg workout
if you can find it and i wish i could help but ben pakulski has a video explaining how he does leg extensions, he doesn't lean back against the back pad and pretty much sits upright with his chest out....ever since i've done it this way i feel it's a lot harder to just load weight and sitting forward i find it difficult to ever cheat (to many ppl go too heavy and don't pause but also grab on so the leg extension doesn't pull them upwards)
I Force Anabolic Minion, Eat, Train, Grow
There was a recent thread on here about the detriments of both leg curls and.extensions. I'd give that a read
If you are going to do them make sure:
1. You are finishing the ROM, and not doing 1/2 extensions. The VMO is responsible for the final 20 degrees of extension, and this muscle is crucial in knee stabilization (and often underdeveloped in many athletes/trainees).
2. Do them in a unilateral fashion. You can do 12 reps on one leg and then 12 on the other, or you can go back and forth between legs (like you would with dumbbell curls).