When I had my 45g, it was a fowlr (fish only with live rock, and sand; no corals) I put in a ball of chaeto in the sump so my nitrates went to 0. So I stopped doing water changes. My ammonia, nitrites and nitrates were all 0, so I figured "Eh, I don't need water changes; just top off w/ fresh rodi water". Sure my fish were fine. But I noticed they didn't look as vibrant; things in there were alive, but just looked like they were living as opposed to thriving. So a few months later, I started water changes again. From the first water change, all of a sudden, things perked up -- the fish looked brighter, my feather dusters and everything looked lively.
So I would have to agree with everyone above. There is something about giving the tank a nice boost. There's nothing like a nice burst of new water to liven things up!
I can't explain all the science behind RO/DI, but I know it removes a lot of the metals and chemicals from the water; essentially making it soft water.