I don't know Toyota's reasons, but plastic (actually composite) body panels
have their strengths and weaknesses. They are more resilient than metal,
resisting dents; they never rust; and they are light. They can also be more
expensive to make and are vastly more expensive to repair - usually only
major cities have body shops that can repair composite bodies of the sort
Saturn uses. On the safety side, composites don't contribute to crumple
zones at all - they just don't absorb energy. That makes crumple engineering
in the frame more demanding.

I used to drive a plastic (well, fiberglass) car - a 1969 Lotus Europa. I
sold it to a brother, who made a U-turn too close to the crest of a hill,
and a station wagon came through the passenger door. The bumper of the
station wagon hit the Lotus frame where it ran down the center console and
bent the frame. If there had been a passenger in the seat there would have
been no hope for survival. Modern cars have much better intrusion design,
but you get the idea.

Mike