Agweb, the Dept of Agriculture's news site, has this commentary on why US farmers should help out developing nations.
I'm going to make two additional salient points.
First, while the US is a world leader in ag technology, and our producers are operating out of some of the best arable land in the world, we don't have a monopoly on food (nor should we) and we don't have infinite resources. As can be seen in the San Juaquin valley in CA (the most fertile valley in the world, not kidding), agriculture is in increasing competition with urban areas for water and land. Even without getting into the climate debate, the raw facts are that an average of X water falls on the US per year. Water is the limiting factor in nearly all agriculture. We cannot produce twice as much food on X acres without a suitable increase in water, and as the population continues to grow, so does food demand and water demand. Helping to develope agriculture in poorer areas not only helps generate opportunity for the farmers there, but helps domestic farmers investigate other techniques, many of which could be applied here to reduce our water impact, effectively increasing the amount of food grown per gallon of water.
Second, we should care about helping farmers in other countries for the same reasons we should support human rights in other countries. Because if we don't help, people sufder and die, without need. We have it in our power to help mitigate that, and I believe doing so is a very natural and moral thing to do.