I don't like using the frame as a ground conductor, especially not for DC currents as this will promote corrosion, and also because a common ground conductor can lead to all sorts of weird interactions between different parts of a system when a connection somewhere in this ground path starts to fail or isn't as solid as you think...
But that's perhaps because of my experience with the more industrial kind of large systems with all kinds of power and signal transmission going on within the same environment - in the automotive industry however, they seem perfectly happy to sacrifice a bit of transparency for saving cost and weight...