During the Olympic Final of the Men's KeirinKeirin one competitor (Chris Hoy) was wearing an aero helmet (red helmet in the picture below), the other competitors wore normal helmets:
The advantages of wearing an aero helmet seem pretty obvious - less drag so you go quicker (Chris Hoy won the gold medal).
However, most of the competitors did not wear an aero helmet so I can only assume that there are some disadvantages to wearing an aero helmet, what are they?