My main use for PP is T-72 hours when I will try to push an upgrade when 3 unsold seats are left. To me that is the best GS benefit.

I don't generally bother to use PP ahead of T-72 because, as has been mentioned many times, they do not clear, and if you get a battlefield upgrade the points are deducted even though you may not have needed to use the PP (although maybe the PP gets you #1 when there are other GS on the list who did not use PP). I also have had better luck keeping the PP-upgrade (when I get it) in case of taking an earlier flight at the gate, but no guarantees and it depends on the gate agent (had a great experience at IND last month).