If you say that the prospective hirer can be pilot in command then you can not say who the passenger that they carried in the front seat must be. The loading and everything else with regard to the flight is the responsibility of the pilot in command and they can quite rightly decide not to carry anyone............after all you have permitted them to be pilot in command and the decision after that is their's and their's alone.
The problem with such simple thinking is that the checker lacks credibility and if the person being checked disagrees then it is a 50/50 argument.
Sorry DFC, it is simple and you are confusing it with various barrack-room lawyery sounding nonsense.
An owner or operator has some discretion over the conditions under which they allow an otherwise legal,current,insured pilot capable of acting as PIC on their airplane to fly their airplane.
One of those discretions is to require a flight in which a 3rd party accompanies the pilot.
If the pilot thinks the 3rd party "lacks credibility" then he should find another operator with a credible 3rd party and fly their aeroplane instead.
If there's an "argument", it's not "50/50" - it's down to the operator's delegation of the decision to permit the pilot to fly the airplane unaccompanied to the 3rd party. It's the 3rd party's decision, which I guess could be disputed by the pilot with the operator.
Yes the pilot is acting as PIC, but the operator can impose a condition underwhich he is permitted to take the airplane only with the 3rd party accompanying him and under prescribed operating limits - eg. within the circuit. (you think all those airline PICs could "quite rightly decide not to carry anyone" and fly the 737 off to Ibiza without the pax?). If the prospective PIC disputes this condition then he has to find another airplane to fly.
I am not saying that it isn't better for a 28 day check pilot to be an instructor - I think it probably is. But I can also think of scenarios where it might be fine for them not to be. I just don't think the "is it a good idea or not" debate needs to be obfuscated with the PIC stuff you posted.
rgds
421C