Swinging on a good plane works for most people.Swinging overly flat works only for a few. Which one would you teach?
You can be very flat and be on a 'good plane.' You can be very upright and be on a 'good plane.' I
suggest flatter swings for most amateurs because generally it's more difficult to come over the top with a flatter swing. In a very general sense, I believe that the rear elbow gets nearby the rear rib cage on the downswing. Many amateurs struggle to get that rear elbow in a good position on the downswing. And I believe it's a case of getting that rear elbow too far away from the rib cage on the backswing and they simply cannot return it near the rib cage on the downswing. Couples and Nicklause had that rear elbow far away from the rib cage on the backswing but they could return it properly on the downswing time and time again. I see a lot of amateurs who cannot do that (myself included). So I think it sort of 'uncomplicates' the issue if you can just keep that right elbow near the rib cage on the backswing.
Of course, I wouldn't force feed that down any golfer's throat. For example, a guy that plays in my Saturday group came to me for some help. He was an 8 handicapper and he has a very upright backswing. He was actually hitting everything thin, but we never worked on his backswing plane one bit, mainly because that wasn't the problem.
I work on whatever I think can work for that golfer. If swinging overly flat is what's needed, hopefully I can recognize that and we can work on that. There's one friend that I work with where we found that he needs to swing overly upright (he had an injury that helped cause this), but his downswing path is
very good with this new, Furyk-ish upright backswing and he's murdering the ball.
If somebody is underplane on the downswing, I don't automatically assume it's the flat backswing causing the problem.
3JACK