I just don't think it's necessary. If it were implemented, I'd prefer something simple showing coordinates...
For the most part, specific directions should be up to the leader's communication skills. With a good leader I know exactly what he's talking about. The biggest problem is when they name specific players, e.g. "target x's sector" or "fill y's sector". Generally I know exactly who they're talking about, but occasionally I'll have no idea and spend the entire round hovering over different players to try to find them.
And I disagree that "move to zone A3" would save time. Anyone who has done any serious amount of fort battles can visualize the map and know exactly where they want people to go. Naming specific sectors would require translating what you know to the coordinate system. You can tell everyone in every sector to move west, but if you're naming specific sectors, you will have to tell each specific sector a different specific sector to move to, and know the coordinates of both. Which brings up a different problem with this kind of micromanagement: different sectors are different sizes.