There definitely is a time and place for each helmet. MTB helmets are specifically designed to keep riders safe from impacts you'll normally see in mountain biking, and DH helmets do have specific safety standards for downhill riding and racing. Moto helmets are designed to protect under impacts you'd see in a moto setting; ie faster speeds, bigger impacts, and the ever-present possibility of your bike landing on your dome. The fact of the matter is a bicycle helmet will protect your head better in a slow-speed, small impact because the shell is (generally by manufacturer) thinner than a motorcycle helmet. Moto helmets are much beefier than a DH lid, so small impacts don't cause the foam to compress as much as it would with a mountain bike helmet. On the same token, taking a big fall (such as DH Diva's) right to your head would totally crush an MTB helmet if you hit a rock because the lid is designed to protect against smaller impacts.
I don't think the moto helmet is right for everybody, but for guys and gals going pro speed on gnarly courses such as Sand Point, it does make sense. For most riders though, I think an MTB helmet makes more sense because you're not as likely to see the kind of impact speeds that Johnny Waddell did last year. Riders doing huge drops or very fast trails surrounded by trees or large rocks can benefit from the moto helmet also.
I'm not totally sure why I felt prompted to write this up, mostly just a public safety notice I suppose. This helmet debate got a lot of coverage last year after Johnny went into his coma, and we're already seeing some of it with Diva's post. Basically every rider needs to decide where their level of riding is, and which type of lid is going to best meet their needs in the event of an unfortunate line choice or variable-positioning rock movement. My two cents, anyhow.
