Rondo is on a great contract right now. That is what makes him an asset. Usually teams overpay for All Star talent.
Rondo will not be on a great contract after this one runs out. He will, as you say, get paid. That makes him less of a trade asset. Blake Griffin was much more valuable on a rookie deal than he is on his current deal. Chris Paul will lose value when he signs his max contract. Derrick Rose is less valuable than he was earlier.
If you think Rondo is worth superstar money, then you give him the superstar contract, but that's totally off-tangent and there's absolutely no way you can say that he's a better value than David West (Or LeBron, Or Melo, Or Bosh, Or Wade) on a rookie deal.
I would much rather pay any of those five guys 4 million dollars a year than pay Rondo 20 million, in terms of value as assets. It has nothing to do with them as players. It's all about getting as much out of each dollar when you're dealing with a salary cap.
If you don't get that, I'm not sure what to tell you.
And, again, I like Rondo, and I have no desire to see him anywhere but on the Celtics.