I just had my 4th ACLR and my surgeon told me right from the beginning that an ACL is not particularly necessary. It all depends on what is necessary to you. If your knee gives out everyday and you really want to ski then you are better to have an ACL. If on the other hand, your knee is stable and you feel comfortable then hit the slopes without it.
The one thing he did say was that younger people are likely to suffer more damage in an ACL-less knee (ie. arthritis and cartilage tears) but even then you don't have to fix it if you don't want to.
I skied without and ACL in both knees for a season but I ended up tearing more cartilage and had some laxity so we figured I'd be better off to have knees with functioning ACL's. Unfortunately, I'm too rough and a little heavy so I've had a revision on both knees. Just recently I used my quadriceps tendon for my right knee revision.
If either one of my ACL's blow out again I probably won't bother getting them reconstructed (at the ripe old age of 40 I'll probably need a knee replacement anyway). I'll just ski the best I can without them. That may mean giving up rough terrain but it's doable.
No surgeon should ever tell you that an ACL is necessary because it most definitely isn't. It all depends on the person's perception of stability and whether or not they are still able to participate in the activities they want. For people who do not participate in any extreme sports an ACL-less knee would likely still serve them well. Even without ACL's I was quite able- bodied.
However, I am missing most of my medial meniscus in my left knee and already have degenerative joint spaces (arthritis) in that knee. I had an osteochondral allograft in my right knee when I had my last ACLR 2 months ago.
Regardless, in many cases ACL's are over-hyped.