Fair question, the car does a lot of work on loose gravel tracks etc and rough bitumen, so even though it does get a little bit of shielding via the joint, it is actually still quite vulnerable....in fact on one of the pics you can see heavy scratch marks along its base starting right at is leading edge.
The extended lip of the resonator is also tends tom act like dragging a hook underneath the car... seems to catch on everything, so what can simply flick past the joint, will then catch on the lip.
The other thing the shield does as bonus is to brace the entire end structure of the resonator.