I think skins are becoming more popular with the magic keyboard because it's such a premium product with such a dull and dreary cover on the top and the bottom. The material doesn't really age well and shows grease and such very easily. I'm not really used to that with Apple products, so that's why I'm personally getting a skin to cover mine. I did that with the Smart Keyboard folio and it looked way better.
Im new to the skins thing so it hadn't even occurred to me to get one for the MK itself. I just wanted one for my iPad pro. The back in particular and a clear one at that.