Basically, you want to create the shortest path to ground for current in case of a fault of any kind. If you connect all of the electrical devices, metal enclosures, etc to a single point by use of a green (grounding) wire, then you will give fault current only one path to earth ground. That is why the grounding electrode AND the neutral (grounded) conductor get connected in the main point of service and nowhere else. Everywhere else, the neutral and the ground are separated. That being said, lets say that you install a subpanel and use that little green screw to bond the neutral bar to the enclosure. What you have just done is created a second path to ground that acts as sort of a dead end for fault current. That subpanel does not have a good path back to earth so the fault will keep searching and possibly cause damage to a large part of the electrical system while trying to find the shortest path to ground.