Users should have the option of removing support for MP3 on their system. Not everyone wants a system with patent encumbrance.
Fascinating - but it may not work. This will only disable optional MP3 functionality; but not mask MP3-only packages. Like -gtk doesn't mask gtk+. Since most MP3 functionality is sometimes done through an external library, it may be sufficient to just mask that particular library.
It best work unless we want to deal with licensing issues in the future. And it should be disabled by default.
So you'd like a mp3 use flag which is added as a blocker to any programme which interacts with mp3s? And you want that turned on by default? Adding the flag is a possibility, turning it on by default isn't.
Closing this as a WONTFIX. If there are a particular set of packages that lack the ability to toggle on/off mp3 support, open a bug for those packages. Otherwise, if you're after a particular restriction for licensing/patent issues, you should check into the (proto?) portage support for licensing, and see what can be built w/in that framework.