They are great for causing injuries. Your form is more prone to breaking down during high reps due to the fact that squatting fatigues you much faster than other exercises. By the time you hit 12+ reps, you will be concentrating more on not passing out from lack of oxygen rather than keeping tight form(if using sufficient weight , that is). I learned this the hard way. Why not just do 4 sets of 5 or 6, or 3 sets of 6 or 8, or 2x10, or 10x2 instead? You will be producing more force and using the same amount of weight and reps. Thus, you will not only be gaining as much mass-- you will also be getting more powerful/strong since the bar will be moving more quickly. You'll also be less likely to get injured. Why do sets of 20? You perform 1 rep in PL competitions. I can understand sets of up to 10 or 12 reps on accessory exercises in order to gain muscle size, endurance, strength, etc. I don't think it is effective for the core lifts.