You had to open that can of worms didn't you.
So many issues to touch on here. Yea, its easy for a student to become confused, when going from one teacher to another who may have a different opinion about technique. So on one hand they would do well to stick with one system and let it take them where it will. Me personally, I have not found a single system of instruction that really has it all right. IMHO, the best coaches and instructors have studied many differnet disciplines over years and reached a higher level of understanding that transcends any single cookie cutter teaching or technique system. With that knowledge, then can then interact with a student to produce positive results that make sense for that particular student, hopefully in a way that will not lead the student down a dead end path. Realize, that there are very few instructors with this level of expertise and teaching ability.
From a practical standpoint, instructors have to put on jackets, go out on the snow and teach skiers with whatever experience they have, which compared to the above is generally not much. At that point, its better to have some kind of system to follow. But there are differing opinions about which system is more correct, and to a certain degree there is an attempt to keep these systems simple so that entry level instructors can teach it and students can understand it. In many cases these systems have incorrect principles that are part of the system. From a practical perspective it may not matter so much as the vast majority of recreational skiers seeking lessons are not ever going to mature into racers or all mountain experts. They just want to ski with their friends and have fun, get down through the bumps, feel in control on steeper terrain, etc..
With that practical objective there is a lot of wiggle room for different systems to get skiers to that goal through different approaches. So in a sense, no problem so far with the differences, they are just different. However, the problem IMHO comes into play as instructors embrace a particular system, they begin to feel it is The Bible of skiing. They become believers with blinders on. At that point, they are expressing dogma. As they move into higher levels of skiing, they tend to want to express all aspects of skiing within the terms of their system. Everything can be explained using their system as far as they are concerned. What I have seen is that this is where the problems with each of the systems begin to surface.
yes, any instructor can be dead wrong if they become too stubborn about thinking their system is the answer to everything.
ok, I need my coffee...