What is the functional difference between using a ground wire, and grounding to a fixture?
It makes no difference as to method of attaching the safety ground as long as the connection is:

a) Done so that the electrical wiring code is followed

b) The connection is reliable and will remain reliable

c) The reliability of the connection continues years into its service life

In any case most switches, outlets, light fixtures these days have separate safety ground screws or wire pigtails that are meant to be connected with the bare ground wire of the feeder cable in the electrical box.