Surprisingly, most modern engines still run water heat, even the ones with multiport fuel injection where the manifold just carries air. It is still necessary once the engine is warmed up. It keeps the throttle body from icing up and controls the temperature of the manifold evenly so the mixture distributes evenly.
Don't forget that the manifold isn't the only part of the intake tract- the passages in the head stay pretty cold until the water temperature comes up anyway. Using water means the metal should expand and contract more at the same rate. The choke does the job of keeping the mixture rich enough for the engine until the metal warms up, then the water heat takes over, keeping gas droplets from sticking to the manifold.
