I'm not disagreeing here, but i think it depends on the client's background. i train a lot of people who were never involved in sports, ever! and most of them are as weak as hell. (hell, i was one of them and after 7 years, i can only bench 335 lbs). no matter how much you try to show them how to do the exercise correctly, they just can't because it doesn't feel natural to them. for instance, during deadlifts, they usually slouch on the way down, and they always lock their knees. I tried teaching two different clients on seperate occassions. both started with deads... none of them could get them right after weeks of training. i switched one to hyper extensions, and after developing his lower back a bit better and he felt like having a slight arch in his back as being normal (as opposed to slouching), when he switched to deadlifts, he got them right away.
I kept doing the bench press wrong for 4 years!!! i never rolled my shoulders in because it never felt natural and no one ever showed me how. after my chest grew (weakest body part), i had a better mind-muscle connection and rolling my shoulders in to isolate the chest felt more natural.
If someone played football and worked out to get stronger when he was younger, when he switches to bodybuilding he will be stronger than a beginner which will let him feel more natural and know how to do the exercise correctly.
remember, most beginners feel leg extensions are a natural movement, but squats aren't. Yet, veteran bodybuilders feel the exact opposite.
so all i'm saying is, it depends on the client, and his beginning level.
hope that makes sense