If you would bond a subpanel, the neutral wires would become common with the ground wires. When you draw a load on a 120 volt circuit, the neutral carries the current back to the main panel. This neutral wire will then have some voltage drop to it. (it may not be much, but there will be some voltage drop, depending on the current draw) Therefore, if the subpanel were bonded, the ground wires would not be grounded, but would be raised above ground potential.
Therefore, subpanels should not be bonded, and the grounds should be run separately to a good earth ground.
Hope this helps,