"I have been told by pilots that instructing won't teach you a thing about the 121 world"

Sounds like something Skyhigh would say.

I'd traslate that to mean "instructing is a waste of time if you can get a regional job sooner than later". This is a popular belief. And I'm all for moving up the career ladder ASAP. But not at the expense of putting a well qualified warm body in the right seat. Flight instructing is one way of expanding one's experience level to become a better jet F/O.