The PP is seen by some (me included) as a way to transfer sales from a high royalty outlet (up to 40%, 50% on ELs before they took the 10% bonus away) to a low royalty one (20%). No RC credit for any sales via the PP, so over time, you have a harder time maintaining your higher royalty payments at IS.
It's about monthy return on your portfolio, and SS has been doing pretty well over the long haul with that.