Song roams forward because Wenger allows him to. Do people really believe Song is going on these runs forward of his own accord?
If Wenger instructed Song to sit and just hold in front of the defense we'd look much more solid, but he encourages Alex to get foward.
If we went back to the system of one 'holder' with two ahead of him rather than the double pivot we play now we'd get much more out of Alex.