I'm just not sure what you gain from playing guys like Young, Nicholls, Bruce or the SP. You know what you're gonna get from them. If there was a sudden middle order emergency and you needed to select Bruce, then his appearance or not for the A team isn't going to make you think differently about what his returns are likely to be. Sure, some experience is needed in the A team.
If I look back to that NZ A vs. Australia A series, the only players below 30 were Foxcroft (25), O'Donnell (29) and Duffy (29).
2nd Test, in came Mitch Hay, Abbas, Ashok and O'Rourke but that squad is still suffering from the same maturity bias as the Test team. Of the batsmen in that first test (and the 2nd for that matter) they're going to be retiring or declining at exactly the same time as Mitchell, Williamson, Nicholls, Young, Conway etc. Great that Abbas was there, but Cooper and Solia are not realistically replacements for Latham/Conway.