With this option, you could fire the pop-up flash (very nice for fill flash with a diffuser) as well as a anything connected to the hotshoe (x-sync adapter, wireless flash trigger to off-camera strobes, etc.)
Right now, if you put anything in the hot shoe, the pop-up does not engage. If you engage the pop-up, the hot shoe can be used. It's one of the other. I'd like to see a option where any combination of these could be used.