For a reef tank, I would highly suggest you use an RO/DI unit. Reasons being that it would put unwanted nutrients in the tank which would eventually lead to a major algae bloom. It could also put metals, like copper (from the copper pipes that run through the house), which aren't good for the tank in general. It could also put chlorine and other water conditioners in the tank which aren't good. I know they make think that take care of these but sometimes they dont cut it all the time.
Besides, if you have one and have a big trashcan for fresh and saltwater, you never have to freak out if there is an emergency