Single-Point grounds...
vs localized ground connections is a debate that can go on indefinitely, and there is quite a bit of literature available on the subject, as well as a number of professional career jobs that deal in this world.
The final answer is...it depends. For simple aircraft with no electronics, local bonding to the aircraft structure (lights, electric gyro's, etc) for the (ground) return path is adequate. It may also be fine in cases where more elaborate electronic devices (GPS, glass panels, audio panels, etc) are involved.
However, as these newer, feature rich devices have evolved, their internal design involves the use of high performance solid state devices which can be affected by even the slightest variances of voltage. This variance is "felt" as a difference between the applied voltage (call it battery voltage) and a reference "voltage" (call this ground, chassis ground, airframe ground, or the return line). If for any reason the ground reference voltage changes (ground shift), even though the applied battery voltage is constant, the eguipment will behave as though the battery voltage has changed (close enough for this discussion).
As an example, when multiple com radios are attached to an audio panel, each device that applies an audio signal to the audio panel has its own power line (battery voltage) and power return (ground) lines. If one radio's gnd connection is located on one side of the aircraft, and another radio's gnd connection is located on the opposite side of the aircraft, and the audio panel pwr gnd is located somewhere else, then each device will only sense the "localized" gnd. The resistance that can be measured from one gnd location-through the aircraft structure-to other gnd locations...causes the individual units to "sense" a different ground reference voltage. These differences lead to unwanted gnd currents flowing between the units and can be discerned as an audio "hum" or other instrument missbehaviors.
Single point grounds is the desiplin of bringing all ground returns bact to a common bonding point (relative) to eliminate the differences that might exist in a given application. Even so, this is no absolute guanentee that you will not experience some ground path issues...but it does tend minimize them. And in dealing with the audio panel and the individual audio input signals, it is important to follow the manufacturers reccommendations on signal line shielding terminations. Generally, you do not connect the shield to both devices, but only to the audio panel. This is to prevent the establishment of an additional ground path between active devices which can contribute to ground loop currents.
And Mel, your comment of using planet Earth as an active power return path, well...here is my 2 cents: for the US / North America power distribution grid, we use 3-phase delta, and the rest of the world is 3-phase Wye. Nowhere is the earth used as a current return line to the generating plants. Many state, and local codes, require bonding of the "neutral" leg to an "earth" ground at a building's power feed- distribution panel / load center. This, plus the earth bonding at the power poles, etc, help to establish the line we call neutral as the reference / current return line.
