I think the build of an U21 player is totally dependent on the perspective you're viewing it from.
I'm the England U21 coach so from that point of view, I want someone with as high primaries as possible in his desired position so I have solid options filling the positions on the team. I agree with AirForce that from a training perspective, I'm more interested in ID and RB than IS, I want that trained last if I could have it my way. For a Guard I'd want high OD and then PA for Point Guards and JS/JR for Shooting Guards, as expected.
Looking at it from a different angle as a manager of my own team, I'm ALWAYS training 1v1 forwards for the whole of their first season, no matter the trainee. I think the added bonus you get from the elastic effect is worth spending time building a trainees HA, DR, IS and JS. With the added extra of a youth trainer, you can get upwards of 20+ pops a season, which is a staggering amount. For your own team you also want more rounded players and starting off with 1v1 allows this.