In my case it's more about tactile experience than cleanliness / keeping it looking brand new. I think it's a shame that Apple don't make the texture of their keyboards last. If I'm typing on a computer for less than a year and it feels like the surface of the plastic has rubbed away, that's an issue for me and I'd prefer them to improve it. I know some people don't care but it doesn't mean that it shouldn't be improved if there is a solution.
I found two good work-arounds for what I need. There's a trackpad cover by Spigen called Airskin that I put on today and it has a nice feeling matt surface. I'm probably going to get the stickers that DMG35 mentioned too because that seems like a good solution for the texture issue I now have. Won't stop the dust getting under the keys but at least they'll feel good to type on.
It’s the nature of ABS plastic and virtually all keyboards are made from that material. The material is smooth and shiny, like a Lego block. There more durable plastics but those aren’t as thin as Apple wants.