Now how many flying schools regularly take 'passengers' in the back?!
Practically everyone operating 4-seaters, I should think! I can see the issue with the upgrade situation you mention, but if you're already operating a 4-seater for the trial lesson and the pax joins the flying 'club' and doesn't pay any more money for the flight, then I can't see that it's Public Transport.
We routinely operate students in pairs so that the 'passenger' in the back can get an appreciation of the lesson before it's their turn and I'd say that this is of value to students. The person in the back can only strictly be described as a passenger; they can have no part ot play in operating the flight. Having said that, they are frequently useful as another pair of eyes, spotting traffic and that's been very useful to me recently.
Trial lessons aren't all jollies that should be operating under an AOC, you know. I converted 3 in the past 6 weeks to students who are now training. I treat EVERY trial lesson as a potential student, they all get the Ex 4.1 briefing and I expect them to fly the a/c.
Cheers,
TheOddOne