Just a thanks to all those who contributed info on this thread. I'm learning vocal arranging and have found these 'symmetrical' chords baffling also. Appreciate the 'why' answers as well as the straight facts, it helps one understand.

The Ab and Eb keys are indeed used a lot, that is a problem. I know it's because they are comfortable keys for voices to sing in, but I don't get it either. Seems like we could all write in a few standard keys and shift pitch a few half-steps when necessary.