I bought a big pack of msg from the Asian supermarket and use it instead of normal salt for many things. My partner and I call it wonder salt.
I hear the voices of my ancestors cry in confusion.
But seriously speaking, I've never encountered MSG being used in place of salt. We use it here to give food more of that nondescript meaty taste (aka umami).
Personally, if I need both salty and umami tastes I'd reach for soy or fish sauce first (depending on what's being cooked). I'd only add MSG and/or salt if I really have to—usually to make minute adjustments.