I know I've thought about using them in a game text before, but I've been trying to think of a better way to incorporate them. I think I've found it.
How about putting QR codes into the margins of the hard-copy main rulebook? Then having those QR codes link to YouTube videos that explain the concepts in a bit more detail, either with a very simple visual style or with live action play examples.
The downloadable PDF copy would convert these QR codes into direct web links.
I think there's potential here.