Thinking about it, the chasis doesnt really need to be the ground as the gearbox/engine is earthed & all the other electrics are grounded to the bodywork.
But i would still ground the chasis as the body panels all bolt to the chasis.
So running a "bus" would only work if you totally isolated/insulated the body from the chasis, which would mean non conductive bolts and non conductive sections which touch the chasis. Also along the corrosion lines, to stop corrosion on a Defender you need to get rid of all the ally; or the steel!!!!!