It's believed to be a mechanism for neurodegenerative diseases occurring later in life, like Alzheimers, Parkinson's, and ALS. Especially with a deficit of supporting brain food (omega 3s and glutathione were a few of the things he highlighted). At the time of it's writing (1997), Blaylock states that the available research doesn't fully understand how these diseases occur and that there are also cases of inherited sensitivity to MSG, so he's pointing out a correlation, not necessarily a direct influence. There's a lot more current research on MSG toxicity that basically supports these claims though. MSG is probably one of many causes, IMO.
I have also recently become a fan of going back to the fats I grew up with...butter and steak and all that other awesome good stuff that has been touted as bad for so long. I still moderate it to a degree but I'm pretty sure it doesn't hinder my weight loss. I got fat from eating processed crap, tons of simple carbs and very low fiber. Changing that has made a big difference for me.