I may not be the smartest guy around, but it seems that a plastic spinner breaking into a zillion pieces would be absorbing and dispersing much of the force of that impact. This would lower the forces against the engine and cause less damage.

Kinda like the Nascar safer-barrier.

But, then again ... I'm not a rocket surgeon and the difference would be slight. I use an aluminum backed spinner for strength, but not for protection against that type of crash.