There was a series on the BBC a few years ago (or maybe C4)...called Freaky Eaters.......you would not believe some of the 'diets'. These were people with serious eating problems whereby they could not cope with any other kinds of food.
Supernanny woman had a show on recently where a young boy had only ever eaten Custard cremes (and some Nice biscuits occasionally) they managed o wean him onto normal food - he was lacking in energy and concentration. A beans on toast diet is relatively healthy in comparison, but with all the vinegar in the bean sauce and added salt etc - he is probably storing up an ulcer as minimum.
Many years ago when I was in dire straits financially I found myself with £1 to last me a fortnight. I bought a kg bag of oats and survived on two bowls of porridge per day for the entire time. I don't recall any ill effects, though it was rather bland.
My Mum apparently took me to the doctors because I would only drink Dandelion and Burdock pop. He told her as long as I was drinking something I'd be OK and would no doubt grow out of it. He obviously wasn't too concerned about my teeth! (by the way my teeth are fine). I didn't drink any hot drinks at all until my 20s.