Misterbonkers - I, too, was coming from a private flying perspective. Every time I take a passenger I check their harness and also demonstrate how to undo it in an emergency. I also show them how to operate the door.
Whether private or commercial, it is still the pilot's responsibility to look after their passengers - hence the safety brief. You might think this a little sanctimonious but I take it quite seriously as my passengers tend to be family and close friends !!
I fly a Schweizer which has a four point harness. It is not much different to an old fashioned car seat belt (only with more bits). It has a flip up release so can be undone very quickly in an emergency. Harnesses shouldn't be any more difficult for passengers to deal with given proper pilot supervision and therefore I would suggest that the reason lap belts are fitted is weight and cost alone - with emphasis on cost
Cheers
Whirlygig