Like the title suggests, the main point of this comment is to highlight the fact that paying someone their market value does very little to give your team an edge. In fact, in a perfectly rational and accurate market, if every team paid players exactly what they were worth you would by definition end up with a team that wins 42 games and loses 42 games.
The battle of the salary cap, therefore, is won and lost by GM's who can squeeze value out of players in EXCESS of what they are actually paying them. In the case of Lebron James, for instance, his actual worth in a non-salary cap world would be at least $40 million - therefore, any amount you choose to pay him under the CBA is going to give you an out-sized advantage. That's the reason why true stars are so valuable in this league.
Likewise, rookie stars on their rookie contracts are also immensely valuable for the same reason: the benefit they confer is greater than what the market is actually allowed to pay them.
Then there are players who voluntarily take less than their market value, and players who have been systematically undervalued. The Spurs have multiple examples of this (Tony Parker, who has made all-nba teams multiple times, making only 12 million a year. Manu, whose numbers on a per-minute basis rival Kobe Bryant's, perpetually being underpaid because he plays fewer minutes and comes off the bench. Duncan, who took a discount on the basis of his old age, yet still outperforms the majority of PF's and centers in the game).
The rule of the game is simple: you don't win big by playing fair. You need to show up with a stacked deck, and you stack that deck by getting players who give you more value on a dollar-per-dollar basis.
How does this relate back to the Celtics? Well, we WERE doing a great job of this with Rondo at his current price. But Ainge has managed to blow it Jeff Green and more recently Avery Bradley. It's tough to argue, when you compare other player signings, that what Jeff Green and Avery Bradley are getting paid is "unreasonable". They're not albatross contracts. In fact, if anything, they are eminently fair prices. BUT YOU DON'T WIN BY PLAYING FAIR. And Ainge's failure to recognize that will cost us, both in terms of long-term flexibility and in our ability to craft another contending team. Ainge is making the "right" move by giving Bradley $8 mil per year, but it's not a winning move.