It never dawned on me that the rumble seats might be dangerous. I guess I thought most people didn't drive very fast, since a lot of the roads were still unpaved. However, I can see where it would be really uncomfortable to ride on a gravel road in the rumble seat. I imagine gravel would fly up pretty regularly. That wouldn't be at all comfortable, having gravel flying in your face.
I've seen cars with rumble seats at car shows. I always thought they looked really cool and fun. They might have been if you were driving slowly around town, but not so much on an unpaved road.