The problem with using wires for return is that if the devices in question also contact the chassis (and most do), you are creating ground loops, which tend to create electrical RF noise. Bad for audio systems. Doesn't matter much for short distances such as engine-to-chassis bonding, but running a separate ground wire is not best practice for long distances. The chassis itself has huge ampacity anyway, so it is best to just use it.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)