Thanks for your replies guys. I suppose the logging of hours isn't the real issue. Here's the scenario. You want to so a x-country to another airfield. The students has done a few solo x-countries. PPL flies one leg and student flies the other and therefore the cost can be divided in whatever is agreed to be a fair split.
Even though the student may not be able to log the hours, surely there would be valuable experience gained for the student.
For me it's the experience of flying, not just clocking up hours, that I enjoy so much.
Cheers.