I'm the first to criticise Eurocrats, but what's wrong with the basic concept that in a single pilot aeroplane you're either commander or under instruction? I've never seen the sense in the FAA version - expedient for certain people, but not particularly clever surely. I just can't see why, save possibly in a test, PICUS is anything but "student" ?
My understanding of the history of the regulation (
91.109) is that it was introduced to enable people to be able to rent a multi engine piston solo, but insurance companies wouldn't insure someone solo with such low hours, so the FAA introduced a way to let students log pilot in command happen to meet FAA CPL requirements.