Duxford's entry states that everyone has to telephone for PPR and a special briefing and it doesn't matter how many times you have visited previously it makes a lot of sense to do so as they often have their warbirds practising and I for one would not like to get tangled up with them.
Personally I dont think it makes any sense at all.
If you happen to be passing by, and they are not practising, or as is often the case, there are only a few visitors, why should the tower not clear you to land? If they are practising, they can of course tell you sorry not today I am afraid, or, we have aircraft in the circuit, are you familiar with our procedures? It really isnt rocket science as proven by places such as Headcorn where you can pitch up most any time with parachuting, aeros in the overhead and plenty of other things going on.
In most other countries this does not go on, and I suspect would not be tolerated. When it gets to the points that you cant fly any where without filing a flight plan, notifying customs, notifying the police, 'phoning to tell them you are coming (and going), asked not to forget your yellow jacket etc I suspect most people will have hung up their headsets permanently.