Think of your motorcycle helmet in much the same way you do a haircut. In the summer, when you're riding through heat waves shimmering off the highway, it always feels better to have as little going on up there as possible. That way you can enjoy the feel of the air on your face and get the true essence of a motorcycling trip. But, as the winter rolls around you're much more likely to let your hair get a little longer to help keep your head warm. And, because heat escapes from your head, it's a good idea to protect yourself as much as possible.
That's why a full motorcycle helmet is such a good idea come winter time. While the full helmet is going to keep the heat from your head trapped and thus keep you warmer, the face screen will also protect you from the harsh winter winds you'll be riding through. So, while you might have to give up a little bit of freedom, knowing that your winter locks help ward off the cold should more than make up for it.