But the pilot must pay his share,
I think you have missed the point, there is nothing to share because there are no passengers on board and cost sharing is an exemption that relates to the carriage of passengers. No PAX NO COST TO SHARE!
Doesn't that mean that one can use a PPL holder to ferry an aircraft, with him paying nothing
Yes, one club I was in did it all the time. One well known company used to do that every weekend for years when PPLs ferried the aeroplanes between the Commercial Training base where they were used all week and the PPL schools where they were used at week ends. The benefit you refer to is irrelevant as again it has nothing to do with the carriage of passengers i.e. Public Transport
Put it another way, I am given £150 for my birthday, I spend it on flying, no problem. My brother is given £150 for his birthday, he gives it to me to take him flying, its illegal. If I took him with my £150, I can get him to pay £75 towards the cost so we have a beer as well!