As others have said, closed face mostly for debris and also to add lower face protection in the event of a crash / airbag deployment. I have both and tried the open face during the last track day but had a couple scares with rocks coming in and will stick with the full face from now on.
08 E90 | Jet Black / Black | 6MT
Mods: RAC RG63 w/ Pirelli Corsa 255F/285R | Akrapovic Evo Exhaust | OEM Blacklines
Winter Setup: OEM 18s w/ Nokian Hakkapeliita R 245/40-18