Second, if you really wanted to see better divers, get them away from the DM/instructor led dives, and let them start planning and leading on their own. Continuing to push supervision when it's not needed only results in dependent divers, who are less competant.
I think at first it's needed. A recently certified diver hasn't learned navigation, rescue skills, may still have problems here and there and need a helping hand... and supervision allows them to feel safer and progress with their skills. But they should be pushed to start diving independently, not stay all the time with a guide.