Why do you need one?
If you are only in piste and don't do park or pipe then I cannot see the point?
Snowbording yes piste sking no.
Are you telling me you can't injure your back on a piste???
EVERY person i've ever seen stretchered off, or helicoptered off was on piste. I've been skiing for 24 years and have seen plenty!
Back injury is not uncommon for skiers and wearing a back protector is a smart move.
I have a Ski specific Camelback which affords some protection, but not a great deal.
I have worn a helmet after being forced onto a huge slab of ice by a novice. I wasn't going very fast, but it was my fault because I hadn't been watching her properly. I'd assumed she was a better skier, but she turned unexpectedly and the resulting outcome was me nearly knocking myself out.
I sometimes ski fast, as much as 60mph on occasions, so I should be wearing back protection. I do this on empty pistes where I can see the well ahead, but it's still risky.
Accidents can happen anywhere, to anyone, with any equipment and wearing kit to mitigate injuries is a very wise move indeed!