I was looking for vids of the mammoth downhill course since masters nationals is there next year and 2:22 in the vid below made me think I should really get a back protector...
http://discovermammoth.com/videos/masters_ski_racing_mammoth_mountain_video.html
The first racer should not have been permitted to leave the start with that face guard. Totally illegal.
I think the benefit of a spine protector without a neck guard of any sort totally outweigh the risk of no back protector at all. I've never hit b-net (knock on wood), but I've hit gates after falling and poles hurt; both gates and b-net poles.
Edited: I hope to make it to Nationals this year. Woo hoo!