I think Matt hit the nail on the head,sort of, On the newer 4WD vehicles a small amount of tire size difference will have adverse affects on the transfer case (i.e. tear the hell out of it) and when I say small I'm talkin as little as 1/2inch difference in diameter will make driveline make weird noises.... And on the passenger cars with anti lock brakes & traction control you have wheel speed sensors so all tires hafta be very close in diameters or you throw the sensors off.
I don't think they are talking about our hotrods at all.........and the vehicles they are referring to would be 1991 and up...