Yep that makes sense.
If you weld your armour into a unibody and the armor gets damaged you are stuffed, especially if its something like Ali or cheap steel that most machines now can cut with relative ease.
With the Box chassis you can bolt on thick layers of light weight material [Wood, HDPE etc] and keep changing it, in the knowledge that its fairly cheap and you are saving your main chassis from getting twisted.
[This is coming from the team with 2 unibody machines ]