In my humble opinion....
The 'trial lesson', or what we call it in the trade - excercise three is actually part of the PPL syllabus where the prospective student can have an experience of the sensations of being on board and in control of a light aircraft for the first time... The Air Experience Flight.. Some people are bought these as gifts etc..
I normally approch this with the normal safety brief but no explanation except for a brief description of the controls etc. I usually brief the prospective student on the actions for takoff then talk him or her through it as they do it. They will do as much flying as they want during this 'lesson' and I will normally attempt to get them to do as much of the landing as possible.
This flight is usually any flying clubs front window and if undertaken as above can't see what the problem is. You can't expect prospective students to indulge in spending money before they try the flying out - god knows they may not like it!!
As to CPLs doing it without FI --- don't think so!!! Spend the money get and FI!!!