Centerfield

Well, when you throw in the subsidies....

This is actually not that new. There's been similar talk in the Bay Area although this is nicely thought out. Too me the biggest advantage is the reduction in transportation, itself a large energy black hole.
Tully's right about subsidies. They would need them, but we tend to subsidize the wrong things (like oil companies and corporate farms that have burgeoning profits).
I do see some problems. Each one of those agricultural high-rises needs to be a good enough distance from the other and other highrises to get maximum benefit from insolation, especially at higher latitudes. You can't plant one smack dab in the middle of a lot of high-rises. One idea could be that for buildings behind the aggie towers to have movable mirrors on them to focus more sunlight onto the shadowed side. Of course too many pointed in the same direction and you could have a solar blast furnace.