A possible use case could be upload/download large files
Already implemented it in my uploader, but after hearing reports of a memory leak in Chrome I may need to remove it again. Hopefully it gets fixed soon.
Another thing is that the fav icon is too small to use for any meaningful notification.
It feels like were devolving to the old websites of flashy and blinking images trying to lure people in. Don't get me wrong though, it is neat! But... Where is that disable favicon extension now?