NigelOnDraft, DXWombat, Deeply Concerned, Sammypilot, xrayalpha, Ghengis and chevvron suggested between them 12 different reasons why the airfield might want you to have PPR. Reasons that relate to potential risk, inconvenience or cost (to others, as well as to the happy-go-lucky pilot that comes swanning by).
Reasons suggested against asking for PPR include that they might not answer the phone (Doh!), they might not speak English (I'm told many do in the UK), forgiveness is easier than permission (not really - if you cause danger or inconvenience, they probably won't forgive you!), and that it makes Katamarino laugh.
Very finely balanced argument, I'd say.