I was thinking about doing this also.
Talked to a reputable installer and he says that while it will reduce the amount of pitting from sand and rocks, it will scratch much easier than glass.
You have to be carefull when using your wipers when the windshield is dry and dirty.
2009 M3 Alpine, carbon roof, Akra slip on.
2011 335ix, Le mans Blue, Black Dakota, M-sport, PE Exhaust, VMR VB3's, ZCP 359m's, JB Stage-2, H&R Coilovers.