Entity framework 7 is planning on removing .edmx files from the next iteration. The migration over will probably be tough for people but they will end up with a much better solution if they start relying on the code first methodologies.
Luckily I started with EF 4.1 so I am very used to using code first on existing databases as well as new ones but this will be a giant leap for some developers that haven't updated their skills in the last couple of years.
Obviously there is overhead in maintaining two different model formats. But aside from removing this overhead, there are a number of other reasons that we chose to just go forward with code-based modeling in EF7.