I'll give you a situation that I found myself in night before last.
I work a lot of hours the last week of the month, and a lot of times my tank is slightly neglected during that time. I thought I had a jug of water in the garage waiting incase I needed to top off. Turned out I didnt..I had used it and forgotten about it. The tank was blowing bubbles and I had no top off water...and its 9:00 at night. My tank will not go all night with out the pumps on and still have the fish alive in the morning. Luckliy, I had a large extra pump that I put at the top of the water and ran, agitating the water a lot, to oxygenate the water. Everyone did fine. But I would not have had that problem if I had hooked up the R/O unit that was sitting in the living room.
My point...its worth the investment.