Just using stainless does not mean that shutter quality is always good. Aluminum shutters cost slightly less than stainless but have significant performance risks. The material is relatively inelastic and is susceptible to permanent deformation or even fracture when it is bent. Since bulk diskettes are not protected by individual plastic bags, shutters are exposed during handling or shipping. Aluminum shutters are easily sprung by mishandling or when disks slide against each other.
Plastic shutters offer substantial cost savings.
The better plastic shutters can perform equal to or better than stainless. Plastic shutters can be highly elastic and therefore return to their optimum shape after bending. They conform snugly to the shell without applying pressure to it. Static charge and debris on the plastic shutter does not seem to be an issue since the plastic shell has a much larger area. The most critical location for debris seems to be on the portion of the shell within the closed shutter.