I was told many years ago it was a cost thing- steel plugs were a lot cheaper than brass, that's why they used them from the factory. (every cent counts!). Clean the hole and plug edge, apply Permatex, and install plug. We call them Welsh plugs, and they work- never found a Welshman hiding in my block. If it ever gets cold enough to freeze the water in my block here, I'm heading further north!