As far as the minimum gap distance goes, I think GM kept it a safe distance to accomodate the average driver. Some drivers might be older, with slower reaction times, some may be younger, distracted, with even less reaction time. Better to play it safe than sorry, due to the pervasive liability issues that abound. Hence, the ability to overide the distance with your foot for different conditions/drivers.
I really like the DSCC for long-distance freeway driving, but in freeway commuting traffic in CA, where traffic goes from seventy to zero and back again in about as much time as it takes to type this sentence, I don't use it.