I wrote quite a long reply to this and then realised it didn't make a lot of sense. Clearly I don't know enough to explain it properly, so i'll just give an example:
Ripping one day in bad vis, run right into where a cat has been piling up snow and left behind a huge mess of ice bolders and ridges. Hit one ridge, get bucked into air skis first, land on my back on the top of the next ice ridge. Could've been bad, but thanks to spine protector just a little shaken.
I see them being more useful in the big mountain rather than the park. Where if you fall you could be in a situation where you might impact something hard directly on your back.