personally, I hope they release a usb dongle (affordable) that enables this feature on the 5d MkIII
I'm sure the 5d4 is already in the pipeline with the rumored large ff sensor upgrade, maybe they'll add wifi to that if it's widely accepted by then and the apps are out of beta.
But just now, many "pro" customers might see a wifi control as a reason *not* to buy the 5d3 because the marketing is directed purely at amateurs like "print w/o cable" or "upload shots of your friends and family to facebook".
But...all of those features are useful to pros, too. No more pesky cables for models to take out, I'll be able to give a tablet for the client to see the images as the come aswell. Also, someone mentioned image download and edit while the camera's still in the bag. That would be really useful while in transit. I'd love this feature in a 5dmkIII without having to buy a £700 box that stops me shooting in portrait mode