As this BusinessWeek column puts it, "Brash, bold and abrasive, Rahm Emanuel...is opposite of Obama's cool; he's fire and passion backed with relentless drive. For someone of Obama's temperament, Emanuel is an ideal chief of staff."
According to the article:
"Emanuel's selection demonstrates how leaders need to surround themselves with people who complement them, not replicate them. Emanuel's not afraid to ruffle feathers so he can play the heavy when Obama needs someone to twist arms, one of the ways to get things done in Washington. Every senior leader needs someone like Emanuel, perhaps not with the title of chief of staff, but with the power of it. And the power to drive things forward."
It's common in for head coaches to put together a staff of assistants that have different strengths, traits, personalities, and skills than themselves. It provides the balance necessary for success.
For example, earlier this week I saw an article about how new USF coach Rex Walters gave careful thought to who his assistants would be, saying: "I'm not a great defensive coach. I wasn't a great defensive player and my teams have never been great defensive teams. So when I was hiring the staff, that was the one thing I really wanted to home in on."