This guy is so interesting...
Would you hire a convicted pedophile to work at a day care center? Would you hire Bernie Madoff to manage your investment fund? The answer is almost certainly no to those two -- but you might hire a convicted bank robber to consult on bank security.
Which brings us back to computers. Hacking is primarily a mindset: a way of thinking about security. Its primary focus is in attacking systems, but it's invaluable to the defense of those systems as well. Because computer systems are so complex, defending them often requires people who can think like attackers.