Some airfields have strict movement limits hence by landing without PPR you may 'annoy' them; with other airfields, a local planning condition may require the airfield operator to enforce PPR.
If you land in emergency eg deteriorating weather, you should get away with it, but it is best to observe PPR requirements.
Having said that, the point I made about max movements may apply. I know of an airfield which can only accept a certain number of movements at weekends; one sunday a military radar unit tried to handover a bizjet which wished to div in, not liking the weather conditions at his intended destination; although he'd nominated this airfield as an alternate on his flight plan, he'd not got PPR so he was refused landing permission (by the airport authority not by ATC) as his movement would have taken the airfield over the planned limit.(they'd aready granted PPR up to the limit)
Look in the AIP (not Pooleys/AFE/Bottlang) to see what it says about PPR; I know that for instance Farnborough are very strict (due to local planning controls); their AIP entry says you mustn't nominate Farnborough as alternate on your flight plan unless you have PPR; this may apply to other airfields too.