The short answer is: MSG is bad for you. Still, you can forgive some for not yet knowing this as the evidence of the negative effects of monosodium glutamate have only come to full fruition within the last 10 years.
A meta-analysis published in 2006 (Freeman, 2006) still maintained that, much as a study published in 2000 in the Journal of Nutrition (Walker & Lupien, 2000) had also found, there was no clinical data to suspect any real negative effects. Evidence of its danger only started popping up in 2006, first emerging in a rat model showing real liver and brain damage following a dose as low as 4 mg/kg, and damage being mitigated by vitamin C (Farombi et al, 2006).
2006 appears to be the year when the research switched from showing inconclusive effects up to 30 mg/kg to clear negative effects by more plausible 4 mg/kg doses. In 2008 came a study in the Journal of Autoimmunology reinforcing the findings showing liver damage, and also indicating a role in contributing to obesity in a mouse model (Nakanishi et al, 2008). The liver damage was again confirmed in 2009, this time contributing to non-alcoholic fatty liver disease (Collison et al, 2009). It was more recently found to be associated with being overweight in a chinese population study (Xun et al, 2011)
Liver damage was once again confirmed both in 2011 in another mouse model (Bhattacharva et al, 2011) and a rat model (Eweka et al, 2011), and verified again in 2012 with a feline model (Collison et al, 2012).
Since 2012, MSG has been used in experiments to cause liver or brain damage, as in Horvath et al, 2013, which investigated neural development using MSG to hinder proper development. It is now pretty clear that MSG impacts essentially all mammals negatively, and not just those individuals who are hypersensitive.
To put this in perspective: we have gone from concluding through meta-analysis that MSG has any real negative effects to using it to cause neurological problems in a rat model. Science is not perfect, and it frequently takes years to determine the wide-reaching effects of many additives in our food.
Despite the new science, the FDA has still not updated their stance since 2012, and maintains that MSG is generally safe.
Much as aspartame has also emerged as negatively impacting gut flora, it would not be surprising if many other common food additives are found to impact our biology in ways we did not anticipate. This is especially likely since the effects tend to add onto each other, as was shown in a recent study investigating the neurological effects of MSG and aspartame (Abu-Taweel GM et al, 2014).