I'm not saying that it causes injuries (although I cannot see how relaxing while a tremendous load on your back is a good thing), but no one has ever explained to me what the benefit is? I would think the key to improving/increasing your squats, are normal squats, all the way down.
Coach can do it better, but I think it teaches/helps your hips/butt to be able to fire out of the bottom position of a squat, and strengthens the entire posterior chain.
All I know is that I've added them over the last years, using a cambered squat bar after shoulder surgery, and that I saw a definite increase in my speed, jump, and deadlift.
I don't fully relax my back at all, in fact I feel zero discomfort there. I just settle on the box and fire back up.
http://westside-barbell.com/box-squatting-benefits/Plus, with anyone who exercises over long periods of time, it's good to try new stuff to prevent boredom and learn.