As a slightly above average dev in London (and I mean it), I feel that for many big companies the interview process always achieve on thing: to avoid hiring anyone who could really transform it for the better.
I think except in the worst circumstances, people can be worked with positively and if you bring a certain energy to the workplace you can bring people around to your way of thinking and bring out their passion.
Some people will dead weight no matter what, but they can be identified and let go.