No way is that correct! A DI filter performs the final polish.. it would not be a RO/DI unit without it. DI resin alone could filter your water straight from your tap to 0PPM, but would exhaust the DI very quickly. The RO membrane is used to reduce the work for the DI, and is mandatory in pretty much every DI situation. Buy a good membrane thats rejecting 98%+, that way you are maximizing your DI resins life!