I suspect single leg squats do increase squat poundages, but not in the way one might assume. I think they force the lower body to really heavily recruit those synergistic muscles we use to balance during the conventional squat.
A significant portion of our energy during a free weight lift is burned by just balancing the weight, which is why we typically press and squat more on Smiths. The balancing is done for you. If those synergistic muscles get stronger, the rate of increase will probably go up. I can see that effect plateauing within several months time though.
Thats what I was getting at. It wasnt so much the poundage that increased (though there was a fairly significant increase there as well), but my reps went up quite a bit.
These results were only mine, yes, but my teammates, and the 60 other throwers whom Mac Wilkins trains at his academy have shown very similar results. Their weight went up, and their overall stamina during each set went up.
The stability generated by these excercises activates otherwise unused muscle groups and lessens the amount of energy needed to perform a rep on an otherwise heavy weight. Thus making it possible to rep out 10 times what you had previously repped out 2 times.
Now is this good from a powerlifter's standpoint? Probably not. I may not bother as a powerlifter. All the powerlifters I know, at least the old school ones, are doing sets of 2 and 3 tops at heavy heavy weight on their squats, so a rep increase is of no concern.
But would it be good to have some stability that you hadnt had before when you are at parrallel with 1000 lbs on the bar? You bet your ass.
For powerlifters, helpful. for recreational lifters, body builders, and ath-a-letes, I would highly recommend incorporating into your workout. There will be gains.