Oops. Guess I'm the culprit here (as Marion well knows)

Yep, it's not only useful as far as I'm concerned, but invaluable to be able to "read" "mirror images" of what someone else is playing chord wise. Every good guitarist or banjoist or mandolinist and especially bassist I know of (those who can play along with others, never having heard the song before) does it. It's a combination of seeing where someone IS, and then anticipating where they're going, using both aural and visual cues.

Cheers

Rick