The point of a lower gear in winter is that when your joints are cold, they are not as flexible. You are more prone to overuse injuries. By going to a lower gear, you lessen the stress on your cold joints and also warm up your joints a bit better with higher cadence.

Plus, cycling is about power and leg speed. During the summer, you can ride a bigger gear and develop your power. Sometime, however, you need to ride a lower gear and teach your legs actually to move faster. You'll notice after using a big gear for a while that even in a lower gear, you can't spin all that fast (spinning fast downhill isn't the same as being able to spin a gear fast). So the winter is just a good time to work on your leg speed. You'll find that you can ride faster in a 46x18 than other folks around you in a 48x16. Remember that a world class match sprinter will sprint at 150-165 rpm; they don't try to do 90 rpm in a humongous gear because it isn't as fast. So ..... gear down.