In some applications, such as dynamo-powered bicycle lights or torches (the old-fashioned ones with a metal barrel) the mass is truely integrated in the circuit. That is, the wiring does not form a closed circuit with reference to the mass, but the metal of the housing is used as conductor for considerable lengths. See also the circuit in the right below (taken from fig. 2b in the answer to this question) where the volt-meter could have been a lamp or anything else.
Is this good practice, or advisable only in such exceedingly simple circuits? Is it safe to have stronge currents freely floating through the mass? What considerations exist not to do this?