Barrage Balloons for fun and profit

Barrage balloons are a primitive form of air defense, which consists of a blimp tethered to the ground by many strong wires. Planes that fly into them get shredded. By building a ring of BBs around major cities, one could provide a degree of protection to that city from a 9/11 style attack (Or any aerial attack for that matter.) Admittedly, such an attack is unlikely now since it's doubtful that it would work twice. However, the balloons can be useful in other ways too:

1) Cell phone and wireless broadband towers

2) Radio and TV stations

3) Observation platforms (See the whole city!)

4) Wind and Solar power generation (Winds are more constant at altitude, and the balloons could rise above some clouds)

and of course, advertising. You could also put a video billboard on the blimps, so if their real use, (defense) became needed, you could tell the whole city to run like hell at once. They also don't have to be ugly blimps, something that looks like a hot air balloon would probably work just as well.