What you are saying makes a lot of sense. Especially when you think of crash testing. What's funny is that it wasn't that long ago, and still in other countries, humans were riding in the back of trucks without any restraints.Humans are too important of cargo to mount seats on t-slots (t-slots could handle the weight of the seats but once a 200 lb. person strapped in, even a small crash would overwhelm the ability of the T-slots to safely retain the passengers).
Yes, I know, it depends upon the strength of the t-slots, right?. Just know that they wouldn't be designed for that kind of usage. They would be designed to keep cargo from moving, the heavier the cargo, the more strapping (and T-slots) would be necessary. They wouldn't be up to seat mounting standards due to the very high loads encountered if there is a vehicle impact.