There's argument about the phosphates interfering w/ your body's ability to process protein too.
Its crazy.
Here's another way to think of stuff in water when you are dieting, esp for competition. Your body can use water as is, correct? Coffee, soda, crystal lite, all are water-based but your body has to do some processing on them to handle the stuff you mixed in, so not optimal. My trainer actually calls anything a "meal" that requires processing to assimilate it. This is not optimal when you are trying to get yourself down to famine-dry for a competitoin.