I was curious about that too. I don't have Danny do anything excessive but I have always been taught that a major component of pitching was flexibility. It doesn't make any sense to not increase your flexibility, especially in the back and legs when you are trying for that whipping motion.
As for my son, he is 12.5, closer to 13 than 12. He generally throws twice a week during the off season. I have found that him throwing has allowed him to maintain 80 pitch counts in games without any dropoff in his control. The problem arises during these couple of months when the weather can be frustrating for him.
The garage and net idea is a good one except that the garage is actually filled with my two 1963 Thunderbirds. I could do that at the cages though.