P and R are oil proof and oil resistant respectively. Unless you need that level of protection from oil it's a waste of money.

Additionally, there's something to be said about accessibility. The disposable market is saturated primarily with N95 and the reusable market is saturated primarily with P100. Other filters (N99, N100, P95, P99, R95, R99 and R100) do exist, of course, but they represent a relatively minor part of the market and might require ordering from specialized vendors. They're not really something you're going to be able to get reliably at a local Walgreens or Home Depot.

EDIT: That's not even mentioning that for 100 (be they N, R or P) level filters you should really have a fit test in order to ensure that you're actually getting a good seal, otherwise you've really wasted money for higher protection that won't even happen.