A big bulky case is necessarily the only way to prevent bending. In my head, I'm thinking that the aluminum band could add more rigidity compared to a bulkier case with thick TPU and PC components.
I have a theory, and I'm gonna butcher it trying to explain my thoughts of it. Picture a wooden 2x4 post. If you lay the 4inch side flat and bend it in the middle, it will flex. Flip it so the 2in side is on top and bottom, and try to bend it again. It'll flex but not nearly as easily. So if you scale this 2x4 theory down to a bumper case for the iPhone, then it would give you my theory as to why an aluminum bumper could be just as, if not more, rigid than a thicker "rugged" style case.
It's just that not many people have purchased an aluminum bumper due to the fact that aluminum can lead to a slight lose in reception
Good description of how a relatively thin "I-beam" bumper could add structural rigidity. The problem as I see it is how to secure the aluminum bumper to the iPhone. Binding it firmly around the iPhone could work, but then it offers no shock protection. Any TPU layer between the bumper and the phone would be too compressed to absorb shock. Perhaps the way to go is an outer TPU sleeve fitted around an inner metal bumper which is tightly bound around the iPhone, but then it's bulked up.
Here's an interesting design I found on ebay:
They claim it's shockproof but if it screws down tightly the pads will compress too much to do any good. It doesn't hold the screen off a flat surface, either. Oh well, it's almost a good try!