My league used to use SP and RP. We switched because inevitably there would be a few guys who switched, and those guys became really valuable. When every team had one closer slot, but some guy effectively had two because Jose Jimenez moved from starter to closer (I think Jose Jimenez was actually the straw that broke this particular camel's back in my league), it was really disruptive. We switched to all P to say, "There. Everybody can pick their strategy and use 0-6 closers if they want, rather than one person getting a huge break because they had some otherwise irrelevant guy like Jose Jimenez."