According to this TED talk, car seats are slightly more dangerous for kids
than traditional seat belts, depending on the crash type. Overall, there may be no difference at all though.
Either way, it kind of makes the point that car seats are kind of pointless in most instances.
I had never heard this before, but it is interesting that he says that the reasoning for the creation of car seat laws has more to do with emotion and the cries of lamenting parents and very little to do with facts and raw data.
I don't know what the truth is, it needs more research on my end... but he makes some interesting points and the fact that there is a lot of money tied up in testing and car seat manufacturing, I can see why this sort of information might not be welcomed or appreciated by some of those in the industry.
Here's an article on the subject as well as the link to the TED talk.