A lot of this has to do with the resiliency of his age/athleticism, and I don't think it's an accumulative thing at all. The daily bumps and bruises don't "add-up" to anything that he can take or not take, but rather his body, (and most everyone's at that age, if relatively healthy), repairs itself very quickly, and is not in danger of reaching any kind of "critical mass", where it attains a saturation point and can take no more abuse.
It's really not until the forties, where the body begins to take longer to rejuvenate and repair, and where injuries can "pile up" to a point where they begin to affect the overall health of the body systems. I DO think some people are more resilient than others, and just watching Rondo get up and shake off some of the throws to the floor that he does, is enough to indicate that he may be a little more resilient than most, or at the very least, a little more lucky than most, to have not incurred serious injury.
Believe it or not, a great deal of such injuries are affected by the mind as well, and what each person's emotional and mental state dictates as injury. Where one person has been conditioned to assess themselves as "injured", may be a very different level of physical stress or pain than someone else with the very same problem. Don't get me wrong, a broken bone is a broken bone, but some people have a much higher level of resistance than others, and can take much more abuse.
Some may consider the opposite of such as "wimps", or "whiners", when in fact that person may have a very low resistance to pain or stress, and what one person would feel or consider as a minor injury, may indeed be much more serious to another, or at least cause a level of pain which is much more intense to that particular person. That's why it's so important to not assess injuries in relation to others in general, but to assess them in relation to the history of that person in particular, and what they're feeling.
Even family members are a much better litmus to use when assessing injury or pain than say a stranger would be, as there may be a great deal of difference in the stress/pain level of one person in relation to another. Sometimes it can even be to the point of say a muscle pull, that keeps one person from being able to walk, and yet the same level of injury in another barely results in a limp. This is where the fine line between someone with a low tolerance level and a hypochondriac gets blurred, and it's tough to distinguish at times.
I think Rondo is just a very resilient young athlete, and that he may have a very high tolerance level, where someone like LeBron may have a level that's very low, (though I get more personal satisfaction from just thinking he's a whiner and a wimp). That high resistance level is a very obscure thing to assess, obviously, but can be a very valuable asset, especially in the world of sports. Some players are just less prone to injury, like it seems Rage may be, and others get damaged more often and more easily, like a Penny Hardaway or a Grant Hill, whose careers have been seriously hampered by their tendencies to get hurt.
There are many factors involved, and these discrepencies of judging resiliency and injury, while being light-years ahead of where they were even ten years ago, are still too intangible to get a solid grasp on, or be able to assess in any consistent, objective way. Let's just be grateful that Rondo has, for whatever reasons, been one of those who seems less affected by those daily bumps and bruises than the majority, and that his pit-bull tenacity and toughness is packaged in the body of a great young point-guard, and wrapped in Celtic green!