While you do hold on to most of your money renting points, by purchasing you lock in the price of those points. You pay the cash and the price never goes up. I'm betting that the price you pay per point now is higher than the price per point you paid when you first started renting. Twenty years from now the price will be even higher, but if you owned the points the whole time, the price doesn't change.
My experience is that most people wish they had bought MORE points!