Alfalfa is a perennial crop, which helps reduce erosion. It is also a legume, wich adds nitrogen to the soil. It is also a crop that many farmers know how to grow. It's potential as a cellulosic ethanol crop probably depends on whether the hervested material can be transported at low enough cost. Use of protein as a feed source might offset some of the costs involved.
Alfalfa growers want piece of ethanol pie Farm and Dairy - The Auction Guide and Rural Marketplace: "Alfalfa is widely grown throughout the U.S. Corn Belt, prevents erosion, provides a yield boost to corn, and provides the following year’s corn crop with nitrogen needs.
Additionally, the leaves can be fractioned and used as a high protein feed source, while the remaining stems can be used to produce cellulosic ethanol."