I agree--as long as you KNOW what they're doing!
Watching does NOT mean you are seeing or understanding what they are DOING. You are watching and seeing the RESULT of what they're doing.
With shaped skis and properly fitted boots, the signal from the foot to the ski is extremely fast. Even if you did have x-ray vision and could see into the skiers' boots you would not be able to see what they're doing with their feet and weight. You are only seeing the RESULT of what they are doing.
It's like juggling. You can watch all day. You can observe what you think the juggler is doing and you may get lucky enough to be able to mimic throwing and catching three items. But you will NOT be able to really juggle by mimicry because you don't know what the juggler is really doing. The technique is too fast for the eye. The RESULT is evident, though. Mimicry about that could be done but it's not going to be very good.
A few years ago I was having trouble with my XC skate stride known as V2. I asked a friend, three time Olympian, coach of the US team to the junior worlds to help me out. He watched me ski and said I had good balance, good glide, strong legs, and bad timing. "Now what I want you to do is ski right on my tails, stride for stride, and copy my timing". Fixed my skiing in an afternoon.
I agree with NYCJIM 100%. I think newfydog's example actually reinforces what NYCJIM said. Newfydog learned that his timing was off not by watching. An expert pointed that out to him. Then, instead of typing three pages of instructions on an internet forum, his friend simply had him copy his timing. If newfydog had simply watched and followed his friend without any prior analysis or consultation, I suspect he wouldn't be able to fix the timing problem so quickly. Not everyone is so lucky to have hands on coaching. That's why some people buy books, videos, and lurk in the tech forums to improve their skiing. Mine did, far more than possible simply by watching good skiers.
I started skiing when I was 29, and spent a few seasons by trying to copy the movements of thoseI thought were good skiers on the hills. I had a lot of fun and could get down most slopes. But not until I started reading and learning about the mechanics of skiing, and getting MA feedback via the internet, that my skiing really improves.