I hear you, the point is, that simply being manufactured out of plastic/polymer dose not make it a bad part. If it did not serve its purpose in some way that was related to the material it is made of, then i could see peoples gripe.
I can see why some CZ owners prefer a metal guide rod, but this dose not mean that the factory poly guide rod is inherently bad. The poly guide rods have proven to me that they do the job that they are intended to do. To me this means that people generally replace them due to their personal preference and not because it was necessary to ensure the proper function of their CZ.