I was recently told by an endocrinologist heavy into Powerlifting that this was true:
Raw egg whites contain a glycoprotein called avidin that is very effective at binding biotin, one of the B vitamins. The concern is that this can lead to a biotin deficiency. The average male should have an intake of 25mcg of Biotin every day and a high intake of Avidin will shut down intake of Biotin.
Although this can be counteracted by eating more yellows than whites, however not many bodybuilders wish to do so.
The solution is to cook the eggs, deactivating the adivin and solving the biotin deficiency problem.