I think Rondo's defense has been improved this season, in terms of staying in front of his man and limiting his defensive gambles, but I have seen some slippage in the last 2 games (mostly the Indiana game, which was horrible). The people who complain about his defense often don't understand the way the defense is designed.
It's important to remember that pick-and-roll defense invovles a big 'showing' or 'blitzing' the ball handler and then retreating to cover his original man while the ball defender is fighting through the pick and ideally recovering in time to limit penetration or a jump shot opportunity. When this breaks down, it's often a combination of either the big failing to 'show' enough to stop the penetration immediately after the pick is set; or it's a failure of the ball defender recovering enough to stop the secondary penetration or jump shot opportunity.
So, there are two components--the big showing and the ball defender fighting through a pick and recovering. To me, our bigs have been pretty terrible in this recent stretch when it comes to 'blitzing' the ball handler. Too often, the ball handler just sprints right by and gets to the rim. Those plays are on the big men for the most part.
Rondo's troubles come when he sees the bigs failing to blitz enough to stop the initial penetration attempt. When he sees that, his reaction, too often, is to start gambling, the thought being that if the big isn't going to blitz adequately then the only way to save the play is to go for the home run swing--that is, by gambling for a steal. In essence, Rondo sees the bigs losing defensive discipline and uses that as justification to lose his own defensive discipline. What ends up happening, in the end, is that the entire defensive scheme starts to break down.
So, to boil it all down to Rondo not playing defense is to completely discount how the Celtics play the pick and roll.
As Rondo matures, he'll get better at keeping defensive discipline no matter what his teammates are doing, be it when he sees the bigs failing to blitz, or when he sees something on the strong side which causes him to stray too far from his man. His defensive problems, for the most part, are not about a lack of effort or intelligence; instead they stem from trying to do too much and taking too many chances to cover for his teammates' mistakes.