An RO unit doesn't waste "fresh" water - it's waste is a brine that has increased it's percentage of dissolved solids by the percentage of clean water it produces.
If you waste 2 gallons for every 1 gallon of RO water production, the brine concentration returning to the tank will be 1/3 higher than what you started with.
Drawing down the tank further increases the concentration. If the RO unit is returning all of the dissolved solids to the tank, at 1/2 tank the concentration has doubled from the the starting point. At 1/4 tank the concentration has quadrupled. You'd have to refill the tank with distilled water to return the dissolved solids to their original levels - anything else just adds more contaminates to the tank, further increasing the load on the RO unit.
Do this enough times in a row and you can wind up with concentrated brine in your fresh water tank.
Using some non-RO water slows the buildup because you're processing more water through the tank, diluting the brine to some extent. And you pull some of the pull some of the dissolved solids out of the loop as you use the non-treated water.
It's a tradeoff - clean water for drinking, more concentrated brine for your other uses.