As I remember a debate from a couple of years ago, there was a discussion about titanium sliders. Evidently they make nice sparks, but do not resist sliding, so the bike just sparks its way down the tarmac until it finds a soft shoulder and begins the "death roll". IIRC, Joos was not going to seell them anymore but I don't know if they were removed from his catalogs.
It was concluded that the plastic material has some resistance to sliding, thus slowing the bike as it grinds away. I believe it is hoped that the aluminum bobbins will also have "some" braking effect. My opinion would also "NOT be scientific, but I wouldn't think the aluminum bobbins would transfer much more blunt force energy than the plastic.