Snowolf, yea I was going to mention that regarding the spine injuries while wearing helmets. I heard about this as well but like you said who knows which side is correct. I also see that some people have this mentality that if they wear a helmet they can go harder and are less likely to be injured. This is the same mentality that people had when SUV's first came out, thinking they were invincible during the first snow fall of the year. Guess which cars ended up first in the ditch.
I think lower back/tail bone protectors make a lot more sense for beginners and lower intermediates especially than a helmet but then again it depends what riding you do. Like you said in dense trees I'd wear head protection as well.