I wear a 661 core saver when out on the DH bike, nothing for xc stuff.
As said, a spine board (as with all body armour really) will only protect from 'impact type' injuries not hyper-extension/flexion (they may 'help' slightly in this, dependant on design) or compression type injuries, so you could still quite easily damage your spine whilst wearing one.
I fractured/compressed 2 vertebrae in my spine last year as a result of crashing head on into tree, had to have an instrumented spinal fusion, not nice. Since this has happened I've also started to wear a leatt neck brace, did a lot of reading up on em before deciding to wear one tho.
As i've said in previous threads, body armour/protection is a very personal choice and only you can decide what you think is necessary or not.
😀