Going back to logging flight time....I have always applied a simple rule: If I am legally allowed to fly the plane as P1 then I log as P1.
If it's a club or rental checkout I am legally allowed to fly the plane, so I log P1. If it's a cross-channel checkout (for which there is no legal requirement in the ANO or the rules of the air or anywhere else) I log P1. If it's training for (e.g.) a complex endorsement or a night rating I am not allowed to fly the plane without it, so I log PuT and need an instructor in the plane who logs P1.
I have always applied this rule and never had a problem, although clearly others have, and I think rustle is right that it is more a matter of FIs trying to get more time in their logbooks than a consistent application of the rules.