Air Cadets do what it says on the tin
ATC cadets generally join to fly. Its what sets the corps apart from all our other fine youth orgs (OK we have a few Air Scouts but not that many) After 30yrs man and boy I hope I know what makes the average ATC cadet tick and the answer is flying writ large.
As a cadet I flew in 18 different types everything from Anson to Comet R2's
As a squadron officer with 3 different units keeping discipline was a doddle. Just threaten to leave their name off the next flying detail or to 'lose' their application for a gliding course.
As to the original question 'why two seaters' it is that most hope to progess to a gliding course and I haven't come across any 3 seat gliders yet!