Looking at a few of the top teams, here are some players I would say are also not a defensive priority in the 4th:
Joakim Noah, Perk, (maybe) Joel Anthony, and (maybe) Dwight Howard (he gets special treatment to limit his effectiveness so he isn't usually a factor late).
There are other players who are limited offensively but it's masked because they have a PG or playmaker who can get them easy looks at the end of games to keep defenses honest.
When I say Rondo has a unique skill set it's because all the other guys I thought of who were similar to Rondo in the 4th are centers. Their role at the end of games is to find a spot they can hit an open shot from, crash the boards, and stay out of the way. Rondo is our playmaker. He's the guy who gets others easy looks and starts with the ball in his hands. When teams take away his ability to create for others he's dead weight. Our offense isn't set up for him to become a role player late in games. I think it needs to be because Rondo isn't the guy who will win you games late. But he could be the guy who stands at a spot, crashes the boards, and stays out of the way.
Rondo does not shoot like a PG. But on the flip side he doesn't rebound like a PG or play defense like a PG either. He can be an topflight on the ball defender when he needs to be, but his anticipation and ability to freelance make him the type presence on D that's usually reserved for shot blocking centers. He eats up space on the court. That's why he gets so many steals and rebounds, but he also just generally disrupts what other teams do on offense which I'm not sure if they've measured.
So it really becomes what you value at the position. If it's that you think we need a scorer, so be it. But I don't see how that type of player fits with our team as it's currently constructed, and I know you can build a winning team WITH Rondo. It's a lot like Vick (except Rondo puts in the work). He's not in the traditional mold but you can win with him. Can he win a Super Bowl? With the right TEAM he has a shot. The Eagles took that chance with him over Kolb (your traditional QB/PG). Worst case is they'll be a competitive team for a few years and have a number of exciting games. I want the same for the Celts unless you can get a top flight player (Manning, Brady, Adrian Peterson or Patrick Willis and a serviceable QB).
If it's his attitude, I can see that as an issue. I see him as a Kobe-lite pathological winner type. He motivates teammates by demanding perfection, which doesn't work well. But when people say he doesn't care, I don't get that. The guy rolled his ankle late in the game, grimaced a little, but didn't say a word. You could see he was hurt but he still brought it. That's caring, that's heart.