Actually, increased oxidative stress is at the root of several degenerative diseases like asthma, arthritis and Parkinson's disease. Thee is a reason why are body produces antioxidant enzymes like catalase and superoxide dismutase: because reactive oxygen species are fucking harmful to all the cells of the body, especially to the mitochondria. Oxidative stress is also one of the three mains processes driving ageing, the other two being cellular senescence and glycation(cellular cross-linking)
That's because antioxidants from the diet, like vitamins C and E are not very bioavailable. Furthermore, vitamin E can actually be pro-oxidant in some situations or in excess. It is a very long way from your mouth to the cellular mitochondria where the oxidative damage happens. But this does not change the fact that our bodies produce antioxidants for defense, which indicates that ROS are harmful to cells, otherwise the body wouldn't bother. Superoxide dismutase is roughly 10,000 X more potent than vitamin C by molar concentration, and it is directly located at cellular membranes for increased efficiency. A large part of the benefit of caloric restriction in decreasing inflammation and tissue degeneration comes from increased SOD levels in cells.
I am not sure what you are trying to argue here. Vitamin C deficiency is the cause of scurvy, and it is treated by vitamin C. Sailors used to get scurvy all the time, until they started to dress their fish with lemon juice and then they no longer get scurvy. Vitamin C is not a very effective antioxidant "in vivo" but it's deficiency is the cause of scurvy. I am sorry, but I really didn't understand your point here.