Studies have shown you can reduce the total amount of sodium in a product while maintaining flavor by using MSG. I have a bag of MSG in my cupboard. If you come to my house, I'll make you two batches of real chicken stock (ingredients: water, chicken bones, carrots, celery, onion, bay leaf). In one, I'll add x weight of NaCl (salt), in the other I'll add the same x weight of NaCL+MSG. I promise the second one is far more delicious.
Correlative studies: products that include MSG are typically junk food. Hi, Doritos! So, of course MSG is linked to obesity. Study probably also fails to look at sources of glutamate other than MSG (soy sauce, autolyzed yeast extract, etc.)
It gets a bad reputation because it's linked primarily to junk food, or cheaply produced food. That's all.