I just compared his Opponent Production stats vs the other starting point guards in the league. Rondo's first in the league in lowest eFG% allowed and fewest assists and rebounds allowed. He's second in lowest PER allowed and 4th in turnovers forced per 48 minutes.
I went to hoopdata to see the average production for point guards who have played 20+ games and 25+ minutes a game. Listed below are what Rondo allows per48 vs what the average starting pg produces (in parenthesis):
Points 17.9 (21)
eFG% .435 (.483)
Reb 3.7 (4.9)
Ast 8.0 (10.0)
TO 4.5 (3.6)
PER 12.6 (17.0)
He also has the best defensive rating of any pg among the league leaders on basketballreference.
To be fair, he also has something most other PG's don't have...KG.
But your point is legit. The numbers bear out that he is a heck of a defender. However, that doesn't change the fact that he could be significantly better if he consistently dug in, and didn't get caught on picks so easily.
He could go from arguably the best defensive PG in the league (which is not saying much), to being one of the best perimeter defenders in the history of the league.
Now, part of this is not his fault. When he consistently plays 38-42 minutes per game, without a reliable backup, you can't expect him to consistently go 100 mph on both ends of the floor. Its just not possible. And I really hope with the signing of Arroyo (and West hopefully returning), Doc can cut those minutes so he can go harder when he is out there. But still, it is frustrating to watch when you know how dominating he can be when he is locked in.