Trust Enemies?

I once read a book full of wisdom and it said in order to accomplish something, it is better to work with and trust a former enemy rather than a close friend. This is because a former enemy is more likely to work harder, go to extremes, in order to prove their loyalty to you and they are more honest in sharing their opinions about the work you are doing. When I first read this, I was stunned. That’d be like Batman trusting the Joker or Superman trusting Lex Luthor or anybody trusting Michael Myers in a Halloween movie. It wouldn’t make sense! But the more I thought about it, the more it made sense, because I know there have been many times in my┬álife where I am afraid to speak my mind so I don’t offend my friends or make them upset.