use of the mass as in-circuit conductor
The illustration you posted is mine and (a) was for a different purpose but (b) suits your question.


  Is this good practice, or advisable only in such exceedingly simple circuits? 


Yes. Most of the worlds cars, vans, trucks and buses use the metal chassis of the vehicle as the common return conductor for the DC circuits.


  Is it safe to have strong currents freely floating through the mass? What considerations exist not to do this?


At low voltages it is generally quite safe. The amount of metal available in the chassis of a car provides adequately low resistance despite steel's higher resistance when compared to copper. At higher voltages you need to follow local regulations and perform a risk analysis under possible fault conditions.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A simple car lamp circuit using the chassis as a return conductor.

Note that the chassis ground symbol is preferred in this case to avoid confusion with the earth symbol (shown in your question) as the latter implies a connection with Earth which normally is not the case on a vehicle.