PP is just another resource, similar to wood or clay or iron.
If you have an excess of wood, and need clay. Then what do you do. You head to the market and try to find a good deal.
Similar with pp. For instance if the resources saved by using the 20% discount is larger than the resources needed to make back the 30pp. Then you SHOULD be doing it. Anything else is just being stupid (okay, you don't have to actually sell if you don't want to, if you want to just spend pp that is fine afterall your funding the game).
The game has changed, anyone who can't adapt isn't good at this game, and never has been. There have been many things that have altered start-up since the beginning of TW. PP is just the latest version. Being truly good at this game isn't about following a set of instructions, it has always been, and always will be about being able to adapt with the times and with your unique situation. There is no god-build order, or god-income maximizing method(poorly worded but by this I mean maximising pits + farming), or god-OP strategy. Because each of these things rely on a variety of factors (your own activity, your opponents, your area ect...).
In the same way the questline rewards altered start-up, or how active you plan to be(although I find most people never do this - effectively atleast), so does pp.