dive,
the constant on/off of the ro/di will decrease their life.
You can increase the membrane and media's life by a considerable amount if you run them for longer periods all at once. Therefore allowing a flush of the membrane and disposal of the initial product. That way the DI media(resin) won't have to work as hard. A separate container such as reefnut or dburr are using, will provide pretty much the same effect, just that you aren't constantly producing water from the ro/di. However that wouldn't be a bad idea if you were going to be gone for a considerable period of time (longer than what you had in a reserve tank.)
Gator