I'd be willing to bet that if you took to similar skiers and gave them a year, had one do nothing other than ski during the season and had the other ski during the season plus add in other activities like weight lifting, cycling, running etc you'd see a difference. I know that I feel a huge difference from when I did nothing to now when I do lots of stuff. Skiing is easier, more fun, my endurance is better etc. Sure, if you could regularly ski year round one might not need to add other activities but for the vast majority of us that's just not the case.
Resistance training builds a stronger all around body. This is a good thing. If it wasn't valuable then you wouldn't see videos of Olympic skiers doing it.
The stronger I am, the more muscles I have the more my body can do demanding things of all types. I'm not talking about hulk like muscles mind you. 80 lb barbells lifted overhead, curls, triceps skull crushers are very good exercise.
You are probably right. I but I am not a professional athlete and I am not about to model my lifestyle after theirs. I don't care what olymipains do. I don't want to specialise in one sport. I don't see hiking or riding my bike or any of my other hobbies as "training for skiing". I don't see these activities as a quid pro quo. Its not about working hard to earning something. The activity is the reward. Its about finding activities that you enjoy and have fun doing year round. I don't have any intention of doing something that I see as work when I could be doing something active that's fun.
As far as the gym... Those who like the gym will go on their own with out being told, they are there now in all probability. Those who don't like the gym won't go. Or worse they will go and they will waste time while not having any fun. They would be better off doing something that is fun for them to build an active lifestyle.