But for airfields operating under the 28-day rule or other restrictions it's vital that they know who is coming and going so they don't exceed it - or rather you don't accidentally exceed it for them.
I suppose the owner of such a private facility, trying hard to stay below the NIMBY radar, needs to make a decision on whether to publicise it at all. If I had a strip operating like that I would not publish it in any directory for the 10 years it needs to run. I would just invite mates of mine to drop in so I can build up the movements log, for the subsequent approval.
You can't just park in somebody else's drive, so you can't just drop into a strip which you see from the air.
A lot of southern European airports are PPR, Schengen or not. Just look at Greece. Of course there are loads that aren't but it's a pity to throw away the utility of those that are on the basis that one doesn't need to fly there.
I almost never fly to any airport - other than some big H24 ones which I know are not PPR - unless I can contact it beforehand, or the person I am meeting has done so. In the UK it's just a simple phone call.