Is there a significant difference between the new all-mountain/trail and classic cross country helmets?
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......


  The man who found her said her helmet had been pushed to the back of her head and damaged. The chin strap was across her mouth.
  Holmes said he had bought the "top of the line" cycle helmet overseas for more than $200.
  Forensic pathologist Martin Sage said conventional cycling helmets provided little protection from impact to the rear of the head.