As always, there is no simple answer.
If a flight instructor or a very experienced pilot let someone have „a go“ at the controls of a light airplane, at altitude and in non critical situations, I‘d see no problem.
That‘s how aviation careers get started, (Guess how I know ...)
The problems come if the PIC loses situational awareness or gets task saturated while watching and supervising. A typical scenario would be the freshly minted, still very green private pilot, taking up his friends for a ride, being eager to show off. Definitely a situation I would take issue with if such PIC would let someone else „have a go“, and most likely the reason why it is forbidden by today’s regulations.