O/H Departures seem to be a very British thing in my experience - I've not seen them taught elsewhere. Agree with Pull what on the busy circuit and other traffic considerations for a 'Commercial' Departure but will add that not all aerodromes are suitable for proceeding from the overhead in any event: those that sit within regulated airspace and have special entry/exit arrangements or in close proximity to hazards such as PDZ's or Danger/Restricted Areas may be more suited to 'Remote Start' navex planning.
Departing from the overhead is however a pure and accurate form of navigating for an early PPL student and I like it as a teaching exercise, but for the reasons mentioned in terms of time/fuel, it's not particularly commercial.