I think they over paid Rondo. 9-10 should have gotten it done. But it's a case of a bird in the hand, really.
I don't think there would be many teams willing to pay him much more had he hit FA. But the Celts obviously valued him enough that they were willing to pay a little bit more to avoid the possibility that some team would offer Rondo more than eleven. In that situation, the Celts may have had to match or exceed the offer.
$15m/year? Lol! No f***ing way he is worth that now.
All that said. Rondo has a very complimentary skill set for the Celts. His speed, quickness, court vision, and good decision making ability, allow him to "Run" the offense. Also, he knows how to play pretty good team defense. He is very good at stepping into passing lanes and interrupting the opponents offense.
He does not make a lot of turnovers. He makes/creates steals. He is an opportunistic rebounder.
He sucks at the line ~64%. This is not good as most of his points come from driving to the hoop.
He has to work on that.
He may develop the ability to shoot from outside 13". He may not. No big deal. As long as he continues to do what is his primary responsibility. Create offense.