More energy waste. They tend to be hard, lumpy, and rather uncomfortable.
Plastic is all of the above except maybe for lumpy.
And rocks need not be lumpy at all. Flat rocks are easy to find wherever you go. They also have the benefit of being much nicer looking, much cheaper, and more durable. It often tends to be wet. If the ground was wet enough to be a problem, I would guess the plastic bench would be too.