GUEST erbert...

The point of grounding metal parts of an 'electric guitar' is earth bonding, a bit like water or gas pipes in a house. It's not primarily to reduce hum, though it can have that effect by reducing the flow of hum-inducing current.

The reason why you don't do it with an acoustic guitar is because there aren't any metal components likely to be a path to earth.

The danger is an electric current trying to ground itself through you rather than via a more direct path through the amplifier's ground - made worse if you are using a microphone grounded to its amplifier and a separate amplifier for the instrument, each with a 'different potential' (electrically speaking).

You become a capacitor, charge up with current and die!