Soooo...if you are being "checked out" on a brand new type with a 500HP radial engine up front (that is still only a SEP) and you are legal in all respects but never flown anything other than a C150, then it is ok for the RHS "PIC" to wrestle controls from the "sole manipulator" in the event of a botched approach and landing, but it is not ok for someone who is competent on their C150 to sit in with an uncurrent pilot who could also be equally competent on the C150, albeit a little rusty?
Seems to me that it is rather sensible to ask your mate to come up with you and spot you don't make any silly mistakes.....
Anyway I have been on many flights where the only "sole manipulator of the controls" is a black box for a lot of the flight and the PIC might be sat in the RHS........