Yea if home ground only covers a few blocks, its essentially useless. When you're on a run, especially a mission, will you ever be within those few blocks? From the description text, it sounds like the mage has been casting in a given area for so long that they have become acclimated to the flavor of the mana of the area. Why should that only be a few blocks. Maybe Chicago CZ and Chicago Corridor could be separate areas considering the volatile nature of Chicago, but Seattle? Tir?
If its only going to cover a few blocks, it should just be a lifestyle asset, not a character quality.