Here's some comments from Howard:
"Mesquite is considered by many people a junk tree and an aggressive pest, except for ranchers. It now covers over 60 million acres of Texas rangeland. Early in my organic career I was asked how to control Mesquite organically. I told the caller I didn’t know but would find out and checked with one of my rancher friends. The rancher told me that once they stopped throwing money at trying to control the mesquites they realized that they were a benefit on the property. Because they are a legume the grass under the trees is preferred by the livestock plus the animals like the shade from the big trees. The real problem from mesquite comes from futile attempts to destroy it, especially cutting it down, poisoning it with toxic chemicals. The tree grows rather than as a single trunk tree as it originally was, it grows back in multiple trunks in bushy form and becomes a much, much bigger problem. This happens because there are dormant buds that sprout from below the ground line once the top of the plant is killed. Mesquite beans have been used by Indian tribes and are still actively consumed by all livestock."
http://www.dirtdoctor.com/Mesquite_vq896.htm