Spine protectors are supposed to protect your back and your spine from hard hits.
Bruising is one thing, but trauma hard enough to injure or even sever the spinal cord are not uncommon in sports like motorcycle racing (where spine protectors are standard issue equipment).
They don't really do much to prevent hyper-extension because there is usually only a velcro-fastened belt/stomach loop to hold the bottom of the protector in place.
Its padding for slams onto hardpack or other objects (rocks, tree stumps, rails, ski patrol).
Also, it makes you look like a power ranger in the lodge when you take off your jacket.
Read on another forum: "If someone held a gun to my head and said, "You have to move to Salida tomorrow", I'd probably do it. If they told me I had to go to Breckenridge instead, I think I'd just let them pull the trigger."