The pro to getting a Glock "grip" plug is that is gives a clean look, and some argue keeps debris from getting into the void and interfering with the internal workings of the gun. Not sure if that problem actually exists.
The negative side of the insert is that it actually takes away part of the functional design of the gun. The reason the recess is there, is to assist the shooter in being able to securely grab the back of the mag and pull it from the magwell should a mag be stuck or difficult to remove. Insert a mag, and then grab the mag with your thumb in that notch and you will see why it is designed like that.
edit: Johnto beat me to it.