The reason this thread keeps popping up is because there is lack of clarity and proper rule-making from FCL.
Agreed. The PICUS debate is an example. JAR FCL and LASORS just say that under such a such circumstance use P1/S. It does not seem to infer when you may not. LASORS only provides some guidance on successful skills tests and PICUS but there is no mention of this in the rules (which only talk about co-pilots) so it seems clear that it is a matter of what is sensible, not what is written.
I asked the CFI this question. Their practice is as llanfairpg states. The 'posh' school at Blackbushe treat PICUS as BoseX describes.
In fairness, the rules have not be designed around insurance/club requirements for 4/6 week recency checkouts. They have been designed around formal flight training and that is what they describe. But JAR FCL does state the an instructor may log P1 when acting as an instructor which read likes a catch all.
On that basis, I would say that a student logging as PICUS if an instructor has not had to intervene is justified and legal. Certainly the CAA have not questioned this in my logbook.