If you want to find out more about the theory behind insanely large doses of Vitamin-C, do a google search on Linus Pauling. He's the 'father of vitamin c', so to speak.
Pretty much, one of his main arguments for taking in such large doses was based on observations of animals. With the exception of humans, primates, and guinea pigs (interestingly enough), most animals can manufacture their own vitamin-c. We need to get it from the food we eat. While the RDA for a 150-lb man is 60mg, he argued that a goat weighing 150-lbs actually manufactured 13,000 mg per day. Thus, an animal that could produce enough to meet its needs was making 13 grams a day, so this must have been a beneficial amount.
Whether or not you want to buy into this logic is up to you. Myself, I beleive in taking a decent amount of C ( ~3gm/day), but have to question such large amounts. ...a couple more things to consider if you decide to go the hella'-dose route:
1) ease into it. if you start taking 20grams of C per day, right off the bat, you'll more than likely get some bad diarrhea.
2) while vitamin-C is an antioxidant, there is some research to suggest that super-high amounts actually function as a pro-oxidant.
3) If you start taking a lot, and want to stop, taper down. Your body actually gets used to higher amounts, and if you stop suddenly, you can actually get a scurvy-mimicing condition.
...this is just all my 2 cents worth. Do more research and draw your own conclusions.