Trevor Thom also states that ex 3 is not part of your formal instruction, but then sets out a number of important matters over the next few pages. Ron Campbell (do we remember him?) calls ex 3 "Air Experience" and states that the point of it is "to give the student an opportunity of experiencing the environment of flight before undertaking formal tuition in flying". I think that sums up the purpose of a trial lesson perfectly.
I have always thought that if you carried the spouse in the back on such a flight it was a good idea, since what you are doing is taking up another person "who has an interest in the flight". This surely makes logical sense as the person in the front seat is about to make a decision about which a large amount of the joint household budget will be tied up. There must be some sense in checking to see if the other half actually enjoys it or whether they would be better taking up golf or boating (heaven forbid!).
Any thoughts on this view? By the way my memory seems to suggest that there was a sort of CAA view on this expressed in a GASIL article a number of years ago.