Personally I think you have to be a good fighter to fight, no amount of training for someone without that aggressive attitude will turn them into a street/pub fighter.
Mohammed Ali admitted in his autobiography that he was terrified to fight many locals in his home town as 'I was trained to box in a controlled environment, these guys did it night in, night out, whilst drunk, in varying environments using whatever was around them to infict injury. My training would never allow me to compete against these guys in their bar rooms, the same as they would not be able to compete with me in the ring'.
People who regularly live with violence tend to be good at it, folk who train a few nights per week cannot hope to compete.
Obviously for top guys this may not always be true but I reckon good fighters in one particular style are usually naturally good fighters not manufactured.
Compare someone like Mike Tyson and Frank Bruno, natural animalistic fighter versus a trained athlete.