Plastic Rear End

What rear panelling? The entire back of the car that isn't tailgate is the rear bumper cover.

The outermost skin of the Prius is made of steel, for the roof, doors, front wings; the front and rear bumper covers are plastic; the bonnet (hood) and tailgate (rear door) are aluminium. The sides of the back of the car and the door surrounds are part of the steel unibody.

Bumper covers being plastic is normal - they have become progressively larger and larger, on successive cars, until they make up most of the ends of the car. Anything load-bearing is usually steel - the doors do make up part of the crash structure. Only the aluminium parts are unusual - they aren't structural so can be made of a lighter-weight material, but probably plastic would be too easy to break into.

The actual active bumper is a metal bar behind the bumper cover, with an 'energy absorber' on top of the bar, and a significant air gap to the bumper cover. This allows the bumper cover to flex a bit to absorb low-speed collisions without damaging anything. It's standard car construction these days. On the cutaway Gen 3 Prius that I saw at a pre-sales event, the 'energy absorber' was basically just expanded polystyrene foam!

The largest bumper covers I recall seeing, relative to the size of the car, were on the original Ford Ka:

Front:



(the bumper cover goes around the front wheels)



(bumper cover goes around rear wheels as well!)

 

One of the reasons that they make the bumper cover(s) out of plastic is so they can be molded. If they were made of steel, they would be expensive/impossible to draw into shape or they would have to be made of multiple pieces welded or bonded together.

Re: Plastic Hood or Hatch - Ford used to have a FRP (fiber reinforced plastic aka fiberglass) hatch on certain Aerostar models. There are probably other examples as well.

Advantage: Easy to shape. Easy to repair.

Disadvantage: Not particularly lightweight - an aluminum hatch would likely be lighter in weight.