The reason for the plastic panels is this:
http://www.cnn.com/US/9804/27/wheels.beetle.crash/
Albert King, a crash-test expert at Wayne State University, said the Beetle performed well because its bumpers contained energy filters, which work on the same principle as shock absorbers. Energy filters in the Beetle's bumpers compress during a front impact
"This is the first time I've seen zero in pole impact. It's really amazing," he said.
The movie of the 5 mph (8 km/h) test:
http://www.cnn.com/US/9804/27/wheels.beetle.crash/beetle.240.20.mov
They save your lives in an accident and they suffer little to no damage when you bump into poles (to about 15-20 km/h anyway).
thanks,
Andrew