Anti-freeze does more than change the boiling and freezing temps of the water. It has keeps corrosion to a minimum, keeps mineral deposits from forming, and has a lubricant to help lube the water pump and seals in the cooling system. Street bikes should not use water wetter only, unless they like replacing cooling system components. As it was said before, racing bodies use the water wetter for RACE BIKES because it is not as slippery as anti-freeze. Nobody wants a rock in the radiator, have it leave a puddle, and have the next 5 riders biff it in the next turn.
Besides, your bikes are set up at the factory to run at a certain temperature. Exceed, or dip below that temperature specification for an extended time and you increase the chances for wear on your motorcycle.