I think most people seem to agree that swapping seats is not a problem. Now that I think of it, I actually fly at work as an F/O RHS and then for pleasure in my cessan in LHS and then jump into RHS in my cessna for the odd 'intro' flight for friends. As I said I have no instructors rating and don't want one.
If there is something specific about an aircraft, like the PA fuel selector issue, then that needs to be evaluated. But on the whole it is probably necessary development of a pilot to be able to fly a range of aircraft from various positions. Even senior airline pilots (training captains) regularly swap seats depending on wether the art avaluating a Capt of F/O.
However, I would say again, be warey of a 50 hour PPL who wants to jump into the RHS to impress the birds. This person probably needs a little time to become comfortable with his/her machine from the side in which they trained first.
By the way....good point from the person who mentioned that schools offer demo/intro flights everyday with an instructor in the RHS and a .05 hour student in the RHS.
I find there are a lot of people in aviation who create problems that don't really exist. Just be legal and sensible.