Correct me if I'm wrong here, but from a strictly regulatory standpoint, aren't intentional stalls (and spins) approved *only* if the aircraft is operated in the Utility Category? This would preclude the carriage of back-seat passengers during these manoeuvers.
While it is true that PA28 stalls are generally non-events, any aicraft can bite. And having spun a PA28-140 on numerous occasions (it was cleared for spins in the utility category), I would have to say that a PA28 spin is NOT a non-event, and is certainly, umm, more interesting than a C150 spin (though not quite as interesting as my Beech C23's spin characteristics). Moreover it is *not* cleared for spins with anyone in the back and taking the aircraft up to near the spin entry with someone in back is not a great idea. If memory serves it will tend to go into a flat spin with insufficient rudder for recovery. At least that's the urban legend in these parts. I've never cared to test the theory myself, finding spins with only the front seats occupied sufficiently entertaining.
As for being airsick perhaps I'm also conditioned by the fact that the most recent checkouts I've been involved with have been on my own aircraft, and I'm loathe to have to clean out the plane after someone upchucks and misses the bags...
The load check part of the checkout does of course require someone in the back. With regards to observation, perhaps there is or isn't much to be gained, depends if there's an intercom in the back.
BeechNut.