With a metal spatula, you're machining the pan smooth over the course of decades, and also constantly scraping off any good bits that are getting stuck.
One of the things I like about cast iron is that you can use metal utensils that you can be sure aren't melting (which I realize would also not be a problem with silicone).
Not necessarily a sound rational basis for it, but it's at least less gross in my head, which is enough reason.
Rubber spatulas aren't recommended for cast iron. You're much better off with a metal spatula as long as it has a straight edge.