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.
2008 E90 M3 - Dinan Stage 3 and BBS CH's
2000 328i sedan (UUC suspension - still the favorite)
2012 Mini Cooper (wife's)
1991 325iX (sold - gone but not forgotten!)
2011 X3 (Performance Center delivery done May '11)