Would you care to tell us how it is sometimes abused ?
In theory it could be abused if the Police tried to claim information in there was of a highly specialised and covert nature etc and a court believed them (this isn't the same court conducting a trial, but one judging the PII case). In a few cases, the court and insist on seeing the information if they suspect they are being misled.
I do not know in the Bamber case if the court chose to actually see the information, but I suspect they would have.
In terms of what might have happened at WHF that should be kept secret....
A special informant / witness who's identity was at stake (would have to be very special grounds though)
Possible surveillance on Jeremy
If Nevill was a spy!!! and being monitored
If WHF was part of some other big investigation (drugs, corruption, etc)
I'm not sure about some stuff, would need to check...
stuff like sheila was in bed with June and Nev (I think you know what I mean) and the kids had been 'grossly' compromised say.
Excepts of radio comms that might give away details on another operation (say Police Car 12 cannot attend, attending rape in next village)