Racers have been known to use Camelbaks during time trail stages in the Tour de France.

Most road cyclists don't use them because they are unnecessary. The main purpose of a Camelbak is to carry 2 or 3 liters of water on your back. On the road, you can usually find enough parks, convenience stores, gas stations, etc. to refill your bottles enough that you never come close to needing 2 liters (4 pounds). The only roadies I know that use Camelbaks are shorter riders whose frames are too small to mount 2 water bottles.

Note that Zefal (and maybe others) sell 1 liter (1 quart) water bottles the fit in standard-sized water bottle cages.