He has plenty of dialogue in the cassettes, but for me the problem is that he doesn't look, sound or act anything like the Ocelot of the previous games. They don't even acknowledge that him and Big Boss actually know eachother outside of one throwaway line in a cassette.
eh? there's a scene on mother base where he reprimands someone for the way he was shooting his automatic pistol, and said that it was better suited for revolvers... a pretty obvious callback to MGS3
edit: He also mentions about not being able to go out in the field because he still technically works for GRU