The biggest value(production/$ paid) in an NBA market that is capped both for teams and individual players, comes from two main sources:
1. Super stars - because their salaries are capped and usually their contributions are much higher than the salary they are being paid.
2. Rookie contract outplaying their rookie salary. That's why draft picks are extremely valuable. A lot of them outplay their salary by year 3 of the contract. And most rookie contracts are small enough as to not be any type of a detriment to the team even if they bust.
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I think I'm closer to you than to Green on the larger point. But I do sometimes think that rookie contracts are overrated. While it's true that they're a low-risk asset for the team, in the majority of situations they also give relatively low returns (esp. in terms of contributions for contending). In other words, it's not just that NBA teams don't win with rookies, it's that they generally don't win by depending very much on rookie contracts generally.
There are exceptions, with GS, as you point out, probably being the best example, and so I don't want to overstate the case. But while you might get some wins from players over-performing rookie contracts, it's not a very reliable path to championship contention. Championship contenders need veterans (and thus veteran contracts), for the most part.
So of the following 10 teams I consider to have been championship contenders this year (if healthy), I counted 19 "contributors" on rookie contracts (out of approximately 90 total contributors -- assuming a 9 per team average). So their contributions aren't meaningless, but neither are they, in most cases, the key to building a winner. These were:
Atlanta: Schroeder
Chicago: Butler
GS: Green, Thompson, Barnes
Rockets: Jones, Montejunas
LA: Rivers
Memphis: (none)
OKC: Kanter, Waiters, Adams, Roberson
Portland: Lillard, McCollum, Leonard
Spurs: Leonard, Joseph
Washington: Beal, Porter
Of these 19, I'd say that only seven (Butler, Thompson, Barnes, Green, Lillard, K. Leonard, and Beal) greatly outperformed their contracts. In other words, it's not hard to imagine that the other 12 players' performances could have been close-to approximated by a team getting only moderately lucky with a veteran on a similar contract. (Notice also that all but one of those seven overperformers were not freshmen coming out, and half of those six spent at least three years in college.) So out of the 60 players drafted top 15 in the last four years who are still on rookie contracts, only 5 dramatically overperformed their contracts for contenders (if we went to the picks where Butler and Green were drafted, the ratio would look even worse).
Of course there are some others on rookie contracts that could have contributed to contenders if they had been on different teams, but that doesn't change fact the path to contention doesn't typically lie with hoarding rookie contracts and depending on players' overproduction off of those contracts. Golden State is the exception (like OKC was with its drafting "strategy"), not the rule.
With that said, I think those who have argued that the proposed price we'd have to pay for getting Barnes is too great for the marginal improvement we'd get over what we now possess have a good argument. (And I kind of like the options available at 12 this year -- though I wouldn't count on that player being a key push to our leap into contention, with perhaps a possible exception or two; I'd expect the payoff to come after the rookie contract in most cases) I wouldn't dislike Barnes for the Jazz, but the price may be too heavy. And with Batum, the marginal benefit/price ratio looks even worse.
