Because that would fix a whole lot of issues with AFKs, team killers, and trolls in general. Like an example you are playing and some one runs up and frags your vehicle then humps it to add insult to injury, you go into your Destiny menu and see who's running around your shared experience on whatever map or area your in and there's your culprit and you cut your connection.They can no longer see you and you can't see them. Think Fable 3 and the player spheres. Your ride respawns and you go your merry way.
Would you want that ability? There title changed since the new mute option on bnet is being confused with normal mute.
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I believe a feature like that will be in place, though I doubt you can block anyone. You can probably just 'avoid' them in your gamer profile settings. I am a lone wolf most of the time, but I know how valuable team playing will be when descending into the bowels of the hive.