They don't scale very well. It is already in widespread use - e.g., the House of Commons, political canvassing and Speakers Corner. Naturally, a roof top is only necessary for high bandwidth traffic. The PTT's would probably assume this role. The average user would be content to shout in the street.
Most people understand shouting.
Broadcasts are easy. Burning beacons on hill-tops have long been used to warn of advancing Armadas and their like. However, the author believes that beacons may have wider applications than just these. They keep you warm. A secondary use for the disposal of those nasty ISO people.