This is prompted by something I read a few days ago, on the difficulty of caring for school gardens during summer vacations. [Updated: here's the link.]
The food movement, including Mrs. Obama, has pushed for local food in school cafeterias. It's also pushed for schools to teach their kids gardening. Both efforts are laudable; both have a hole.
What's the hole? Schools, most schools anyway, don't operate year round; they close down during the summer. So to develop local sources of supply you're asking a farmer to ramp up production in the spring and fall, and idle the operation during the summer, or find another market. It's doable, I suppose, but it adds in another level of complexity for management.
In contrast if school cafeterias rely on national suppliers and don't limit their requests to fresh food, the suppliers can more easily manage things to provide a flow of food during the school year and direct the flow elsewhere (food processors). Diversification of the market leads to more stability in price and more resilience in response to disruptions and disasters.