Here in NJ, the law will soon mandate closed face helmets. It actually already does but there is a temporary exemption in one case I know of. I've heard about the face shield issue with airbag deployment and this is a concern. Some of us wonder whether closed face with visor removed might be the best answer for HPDE.
In the end, there's always going to be some risk. I use a closed face myself, but I do leave the visor open. Maybe not ideal.
2014 X5 35d (Performance Center delivery May '14)
2015 M235i Cabrio
2000 328i sedan (UUC suspension - still the favorite)
SOLD! 2008 E90 M3 SOLD! 1991 325iX (both great)
2012 Mini Cooper - (wife's car)