It doesn't sound very practical. First you need to prepare your meal before you hit the road, then you have to worry about overcooking or under cooking.
Plus thermos are heavier and bulkier than a stove and i assume you still need a stove right? So I don't really see the point.
And also thermos are probably not easy to clean as they are designed for liquid and not for food. It is already not always easy to clean rice from a regular pot, so I can't imagine cleaning stuff at the bottom of a thermos.
But I never used thermos cooking so this is just my impression. If it is to make life easier, it doesn't seem to be the right way.