Just look at any DS game that uses the mic. There's the point of it. The Wii U controller, and all it's aspects, was created as a tool for game play, a tool for developers to create new games. If the Game Pads mic was always used for cross-game chat, than that would be one less tool that the developer to use. If the developer wants the mic to be used for chat in their game, than it's a simple matter of programming it for that purpose. However, if any kind of cross-game chat is add the system at any point down the line, it would be best that it remain head-set only.
I'm pretty sure that you don't voice chat with someone when playing those kind of games. Look, if it is a moment in a multiplayer game where you voice chat and do voice commands then I'm pretty sure you just don't talk with your friend when doing the voice command.
