Many water utilities actually ADD phosphates to your tap water as a form of corrosion control in the distribution system. Never use tap water if you intend to have a robust mature reef system! Remember, our goal as reef hobbyists should be to provide the best habitat we possibly can for our inhabitants so they can live out their complete life cycle. If we cannot do this we should not be in this hobby. You provide good light, the best live rock, good foods, etc., why not provide the best water you can? Water quality is something you can easily control either by using a RO/DI or buying quality water, its simple and inexpensive.
Water is the single largest ingredient in a reef tank by far and everything depends on its quality and its stability. RO/DI does not change, Tap Water does change all the time. Would you want toddrink the water in New York City or New Jersey right now? Not me!