Hi Rob, welcome to the unhurried overweight mob 
The three point mounting, the powerbeam or the un-tipping tipper mounting are absolutely essential on ANY lorry that is destined to leave the tarmac(except Tatras).
To get the articulation of an off road truck the chassis is designed to twist, and needs to do so freely. There's a great description of why on the Unicat site (link above) in the technical section. If the body is held down as on a road-going HGV the box has to flex and the furniture comes away from the walls.
cheers
Luke
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