A tent in (or even ON) the vehicle is not unsprung mass, so your example is completely irrelevant. You will definitely feel a difference of an extra 12lbs of unsprung mass per corner, but you will never feel a difference of an extra 50lbs inside your vehicle, nor will it (in any measurable way) affect handling, suspension, or gas mileage. Weight up high will play some role in very tricky off-road situations, but considering the number of people running 200lb tents on their lifted, oversize-tire equipped vehicles, I'd guess that's not a problem for the average consumer.
I completely agree that overall vehicle weight should be kept in check, and that lower is always better, but arguing about an extra 50lbs of fabric-related weight on a vehicle with a 1000lb+ payload is a pointless exercise.
The post I quoted specifically compared extreme mountaineering vs vehicle-based travel. While weight plays some role in vehicle-based travel, it is exponentially more important in mountaineering. Some 50lbs of weight is 5% of a load capacity for a vehicle with a 1000lb payload. I'd bet that if folks climbing Everest could sacrifice only 5% of their personal weight capacity for a canvas-based tent, most of them would do so in a heartbeat. I know I would.
