I have been commuting through winter since 2002 and i use a studded front tire. as others have noted, its not really necessary most everywhere in the city. however, I cross the Brooklyn bridge every day and there is always at least one day, and usually more, that snow or rain melts and re-freezes on the wood, making it slicker than an ice rink. on those days, people can't really walk properly and on a bike with studs you can at least make headway. plus for that odd patch of ice here and there, it's comforting.