Some people don't like them. I would be included in that group. Particularly for longer rides and races, I find that my box-section rims smooth things out a little compared to deeper rims. For me, once I spend more than a two hours or so in the saddle, comfort = speed to some extent. Also, we have a lot of wind in my neck of the woods, so I'm skeptical about how much energy is really saved by by the aerodynamic rims when you spend half of the race fighting with cross winds that are trying to blow your wheel from side to side. I do own some V-section rims that I use for crits and time trails. For the pros, however it might have more to do with what the sponsors are paying them to ride.