It would never be strictly medical if it became sufficiently developed. It would be used to see the thoughts of Prisoners of War, it could be utilized by Governments to extract the information they want from anybody it could be used in courts so that the current system used in so many places is no longer in place and it could, eventually I'm sure, be used without your knowledge or consent.
Right now they need to show anybody hooked up to it over 400 images just to be able to get some fuzzy letters out of their head, but in time when they develop a general pattern or frequency to things I'm sure that won't be necessary anymore.
This is a really invasive thing, and something I expect governments to abuse.