I've never seen the engineering data so have to go by 35 years experience in them. 40 deg takes a LOT of power to overcome the drag. It also allows a student to get into some pretty deep sink rate situations that have pranged a lot of 150s over the years. Training to use full flaps at 30 deg keeps things simple for students... 10 deg downwind, 20 deg on base 30 deg on final. They can also easily move between 150s and 152s without altering technique.
Much later in training, after they've mastered landings, you can explore the 40 deg setting for extreme short field ops.
A good discussion on the flap decision can be found here:
http://www.cessna172club.com/forum/ubbthreads.php?ubb=showflat&Number=9290
The fifth post is a good exploration in and of itself. I'm definitely a full-flap landing guy for most landings in most airplanes, but adapt to the nuances of specific makes and models when appropriate. And I teach no-flap landings as an exercise in knowing your aircraft.