The "fingers" method is useful in the very early stages to help them visualise the right attitude in the demo of straight and level flight attitudes, but after that there is no need for it IMHO.
As for limiting the bank angle in a climbing turn, this is to do with maintaining a rate of climb, if they turn crosswind with a bank angle of more than 15-20 in a PA38 or C152, it can lose a significant amount of climb rate, as can be demoed in the climbing lesson.
Agree though that there are a lot of misconceptions that new instructors take as fact. One common one is that full opposite rudder should be applied in a wing drop stall.
and that wings level is more important than airspeed
. Many are taught just to pass the test and have no idea how to teach, many are too inexperienced or immature to do the job well. But we all had to start somewhere, and it's the CFIs job to help them develop in the right way.