There is a new niche of "all mountain" helmets, like Fox Flux, POC Trabec, 661 Recon etc. They feature closed, rounded backward design, and claim to protect the brainstem better. I ride all-mountain technical singletracks with a little of drops and lots of rockgardens, should I invest in a new AM helmet, or there is a minor difference between them and the common XC design?
I would look for evidential based information regarding brain stem injuries these helmets could/would have prevented, preferably published in medical journals, that a standard helmet would not. I think you find it fairly thin on the ground. (If anyone has such evidence, I am happy to retract this answer)
The reason I say this is if you look at a typical/older style helmet, the rear extends a long way back. To suffer a brain stem injury that these new fandangled shapes would prevent, you would need to a hard hit from a protruding rock (or similar), that wedged under the typical helmet. I cannot easily visualize the mechanics of a bike fall that these helmets would protect you against that a normal helmet does not.
A falls that do happen and none of these helmet designs protect against - face plant - need a full face. Stick through the ear - full face again.
One I have seen a photo of is a stick through the eyeball - Helmet didn't help....
Give the limited protection either style of helmet offers for side and facial injury, my guess is they a not much more than a marketing gimmick. Would I buy one - yes, if the premium was not high and it fitted me at least as well as another style, but I would not upgrade an otherwise good helmet for one.
However, I am also not inclined to bow to peer pressure and buy things because they are cool......
Edit: I might have to retract this answer, after reading this report on a coroners inquest into a cycling death......