I do not know the exact answer to your question except to say JB includes them on his "BIG" program. I have also seen powerlifters here include them as assistance. If anything, they are good to get the blood flowing.
I found them to be tough and I underestimated them at first.
My Favorite Part Of Lifting Is Having The Opportunity
Meet PR's: (May 2014) Raw Natural
Squat 475 lbs
Bench: 286 lbs
Deadlift: 501 lbs
Total: 1262 lbs
Squat: 495 lbs
Bench: 300 lbs
Deadlift: 520 lbs/500X5
Total: 1315 Lbs