I ought to just point out one very good reason for their introduction - personal safety. More importantly, perception of personal safety. They were introduced when bus usage in London was far lower than it is today so there was still the perception that the top deck is home to the usual anti-social behaviour (which it is in most places), single deckers were no good on busy routes so some of the key strategic routes got bendy buses to help grow patronage.
Trust me, for all their ills, if they were introduced in Birmingham it would be heaven - there would be far fewer people smoking doobs, playing mobile phone tunes, shouting/swearing etc. if there were no top deck.
I'm not saying this is a good argument given the negative impact on peds/cyclists, but it is a valid one nonetheless, particularly from a bus user point of view.