Not just the Nitrate and Phosphates that are the problem. They can work as liquid fert like EB said but more important are the removal or heavy mineral and potentially dangerous elements.
Good ole fashioned TDS, total disolved solids are all sorts of misc things that over time build with the tanks constant evaporation so they get more and more concentrated and be detrimental to a system.
RO units while not absolutley needed, it sure does cut down on the scary stuff. They are not expensive pieces of equip compared to the benefits the bring to the table.