This was on a box that had no apparent outside damage to it. So in the end I was left with a damaged Mac Pro and the seller gave me back $50 towards a replacement aluminum case.
I've had that happen before. The stance the shipping company takes is that if the outside of the box wasn't visibly damaged in some way (punctured, crushed, wet, etc.) then the fault was with the sender not preparing it adequately for shipping. So far, I haven't had any problems getting FedEx or UPS to pay for damage when it was clear they didn't treat the box well. I also wouldn't personally package a 5xx/5xxx series, TAM, G3 iMac, or any of the other super fragile machines, since all of those can break if you breathe on them wrong, but if UPS or FedEx boxes it up for me and it still arrives damaged it's guaranteed that insurance will cover it since they were the ones to package it. More expensive up front, but much safer later. You either end up with a fully intact machine or you get your money back and maybe some salvageable parts.
It is absurd to think every time to take pictures at every step of the unboxing process, but probably it is just as you say, the more proof the better.
You've got it. With anything fragile I have shipped to me I take pictures of the box as it was received on my front porch, as I open the box lid, and as I take things out of the box. Saved me more times that I care to count.