You're making the mistake of assuming that all helmets are built to pass the standard - which is very low - and no more.* In fact, it's easy to analyze helmet design and ro-monster has done so correctly: other than rotation, the main source of failure is a too weak shell. Shells that are thicker and have fewer vents and protrusions are stronger. So it is reasonable to assume that a skate style helmet will protect more - oh, it's not a certainty, but it is likely. And remember that manufacturers expect users of road helmets never to need them, but skaters and BMXers are pretty likely to crash and test a helmet out, so making a skate helmet stronger makes sense.

Another thing I'd look for for MTBing is a helmet with good shell strength and foam depth at the side of the helmet. The current certs ignore the danger of side impacts, but if you're going to wear a helmet for MTBing they are worth considering.

Final piece of advice (based on reading nearly everything available on line while fighting The Helmet Wars): do NOT assume that a helmet will protect you against concussion! Even the main pro-helmet site says that they won't - the foam would have be at least 3 inches deep to do so. So a helmet won't much reduce the possibility of an injury that will leave you unconscious on the trail. Plan for this and leave details of where you will be riding and when you should be back if you are riding a lonely and risky trail solo.

*Also, tests have shown that most random samples of helmets taken from stores will fail the cert level they are supposed to pass.