Well, I know as for myself, I have personally trained and seen the effects of both JR and SB, so I am not necessarily saying those skills arent important. In fact, a while back I was preaching SB before anyone else and I was basically ignored because I didn't have large enough sample sizes to support my claims. However, with all of that being said, it comes down to the most effective ways for training.
At the U21 level it is most likely quicker to have effective builds simply with the 1v1 training method. If you are talking about a NT player then I wouldbsay yes pleaae for the SB and JR, U21 not so much. In my opinion though, we do need to test more of those playwrs you speak of at the U21 level, but we also dont need a lot of those players being trained in case it doesnt work out. Maybe have a couple owners train these unique builds to test out when they are 21yo with the promise to get some playing time. If it works out then we can focus on training more.