Rondo was a heck of a defender when he first came out, AB was also. Problem is when you start playing more minutes your defensive intensity drops so you can stay on the court longer. Players get to tired or end up fouling out, NBA refs don't call it the same way as college. He guards big name NBA players like that he will foul out in 10 minutes!
I saw an article stating that Steven's asked requested for Bradley to play with less intensity on defense (less full court pressure) and that he wanted him to focus more on team defense.
As for Rondo, I think that's an extreme example. Since the Big 3 got older, a hell of a lot of responsibility fell on Rondo's shoulders. He basically had to handle the ball every possession, set up every play, push transition AND do all that for 36 minutes a game. I think part of his defensive struggles come from the fact that none of Boston's other perimeter players (Bradley, Green) are confident ball handlers or playmakers, and that forces Rondo to pretty much do everything. Part of the reason why I think having a combo guard like Smart ont he court with Rondo would help a lot - I think ht would reduce a lot of pressure off Rondo, because at times Smart could bring the ball up and initiate the offense.