The prototype works, but isn't perfect, mostly due to detection being less than precise. Given that the Kinect, primarily works with exaggerated body movements, such as waving ones' arms around, it's impressive that researchers were able to detect something as small and precise as a tongue in the first place.
In addition to its use as a tool for physical therapy, it's conceivable that such a method of control can open up video games to an audience that otherwise cannot enjoy them, such as paraplegics. A much earlier example can be traced back to 1989, when Nintendo offered the Hands Free Controller for their Nintendo Entertainment System.
The device originally retailed for $120 and can still be found on eBay on occasion, going for between $600 and $800. (Matthew Hawkins for In-Game & DigInfo)