If you could not tell which one had MSG, why restaurants bother to
use it at all? 

If you can taste the difference, psychological reaction might play a role.

The fact is, MSG is part of natural substance. Everyone, I mean EVERYONE,
consumes certain amount of MSG every day through regular diet without
the synthesized MSG additive.

Chinese, and many other Asians (Japanese, Koreans, etc) have used
MSG as flavor enhancer for two thousand years. Do you believe that
they knew how to make MSG from chemical processes? Not. They just
extracted it from natural food such sea food and meat broth.

Baring MSG is just like baring sugar which many people react to.