And if they don't, take action quickly to get rid of them. But do it humanely. Give them a reasonably generous severance package and help in finding a new job, probably through an outplacement firm. Keeping poor performers on isn't good for them or fellow employees. That's good for the employees by making the job more interesting, good for the company because their performance will improve, and good for the managers, who don't have to spend time micromanging those who report to them.
Challenges make an interesting job.
And people who are interested in their work aren't as likely to leave. Such policies are vital in retaining employees. A survey by Ceridian Employer Services of Minneapolis found that flexible work schedules and telecommuting helped keep good employees from leaving. Sit down with employees, one at a time, and tell them you value them and want to keep them. Ask people how they like working in your unit.