Sedayne - Rereading the poem after a great number of years--and thanks for posting it--it seems as though the narrator doesn't really see the value of the fence unless one or the other of the neighbors kept cows. The fact that one grows pine trees and the other grows cherry makes the fence less practical.
                The neighbor, on the other hand, sees a need for the fence even though there is no livestock in the picture. The whole thing seems to go to cause the reader to wonder a little about the neighbor.
                Fast forwarding all of this to the present, relating all of this to the proposed American border fence would seem to depend upon the reader's willingness to equate illegal aliens to livestock.