In defense of the ski harness.....
I am not an instructor. However, I am a mom with 3 biological children, and 2 step children (who I exposed to skiing....they were little when we were married.) All 5 love snow sports.
We purchased a harness when one of my step daughters was 6. Did not work well. She was too old. We quickly decided it was a bad idea.
We pulled it out again when we were teaching (in combination with sending kids to lessons) the younger three to ski. They were 2, 6, & 6. We sent the 6 year olds off to lessons and used the harness with the 2 year old who was too young for lessons. It worked great for us. The one who used the harness has consistently been the least likely of the three to be in the back seat. We didn't know what to have her do with her hands, so we had her put them on her knees. We also mostly used it as back up to not allow her to get into a dangerous situation, often the harness was not taunt. At first, someone would ski in front of her so she had someone to follow. Later, she determined her own line. She was off of it by the time she was 3. She was pulling out of a snow plow at 3 1/2 (please do not tell me this is not possible, because I have already heard this when and I had non local ski instructors apologize because they thought I was the crazy lady over estimating her child.) My point is, with very young children, you can teach them to have fun with the family while on skis. And, it allowed us to be out skiing as a family with all three kids. With the other two, we put them in one group lesson. Then a 1 hour private lesson every couple of weeks when they needed something new to work on. We had season's passes and skied every weekend.
Age had everything to do with the success of the harness in our situation. Personally, I would not do it with a child over 3 or 4.
All three of mine are alpine racers now. The youngest (8 yrs. old) is rarely in the backseat and knows how to self correct in the course when she is (according to her coach). She is an excellent racer. One of the twins is constantly standing up like a tree. He never used the harness. Obviously, the harness didn't teach her to stay out of the back seat; however, it did not hurt her either. (I should mention even at 3 she did not have a problem with weight distribution while skiing, according to instructors.)
Most importantly---HAVE FUN WITH THEM!!!!! The little ones are so fun to ski with. I miss skiing with the tiny kids!