Adam,
A trial flight is classed as a trial lesson, this means the person having the Trial will want to fly the aircraft.
If a PPL pilot was letting others handle the aircraft, well..........
There is a reason why Trial Lessons are given by instructors.
If anyone I knew wanted a Trial Lesson, I would want an instructor on board next to them, after all there is a big safety issue, if you cannot see that or understand that, judging by your last comment you cannot, well...
There is nothing o stop a PPL taking his/her mates or others up and sharing the costs.