I think the big change in '72 across the board was to make everything cheaper and easier to build. Obviously the '72 grilles were much simplier without all the little chrome pieces, bezels and separate grates. In '70 and '71 Mopar was trying to build corporate flagships out of the E-bodies but after that cost cutting took it's toll. Of course the tail lights are another example of replacing very complicated units with glued together plastic units. Cheaper seat covers less options ect.
More examples of that are the cheapie side markers that made their way on most of the cars and even the trucks. The '71 up B-bodies addressed some of the shortcomings of the Es like VINs that are not attached to the dashpads, simpler dash bezels without all the plastic dohickies stuck on here and there and big plastic tail lights.
Why did they change to 4 headlights for one out of eleven years. I think it was a stupid move that just confused people who thought they knew how to ID Plymouths. Mopar could have easily designed a 2 headlight cheesegrater grille but I guess someone in design drew a cool picture and the studio just went with it.
