MSG may in fact be detrimental to human health but its very difficult to quantify this as it is widespread in the food supply, and it is virtually identical to natural forms of glutamic acid in the food supply which are even more widespread.
The situation is somewhat akin to something like wheat in the food supply. Cases of celiac disease or gluten intolerance eventually arise in small segments of the population, but testing for this before hand is very difficult because there are no "control" groups of people to use as a comparison.
Also, there is very little economic motivation to test for MSG intolerance, as there might be for something like cancer or AIDs, so determining if its bad will be a long slow process that may never reach a definitive answer.
Cases of people who have shown a intolerance to MSG do exist, but they are not scientifically validated at this point. However, that's why its required in labeling laws. Its the best the government can do at this point to offer people protection, by giving them a choice to avoid it.