Jack Welch, when he took over GE started the whole 10% of your under-achiever hog-wash.
Now, if you have poor achievers in your company, who have been there for decades - as GE had - perhaps a 1 time 'cleansing' is necessary. But, if you are doing your interviewing competently, and are teaching and mentoring your new hires - to continuously 'fire' 10% of the workforce is not only stupid, it's counter productive.
Consider, how long does it take a person to learn his job function and all the nuances that take it from being merely fulfilled, but where he can then magnify it? Given the proper motivation, a below average performer can become a top-performer. If a person knows what's expected, is shown how to do this, and is encouraged - he will either refuse to conform (termination case) or he will improve. I've seen this, I've done this and it works.
Other employees see this, and morale improves. People do not want to leave that group/company. Motorola USED to be like this. When Samsung came into town, they had to offer 20%+ salary bumps to attract Motorola employees to leave. Why? Because the people at Motorola knew that they were 'safe', that they had a career and a future with the company. Then Hector Ruiz came along and killed Motorola, before moving on to AMD and killing them.
I do not subscribe to the 10% cull; because you very quickly come to the point that you are cutting good people, and replacing them with good people who you will fire in a year or so. This creates a hostile work environment (why should I welcome you, help you or agree to work with you - if I'm competing against you to keep my job?), slows projects down (people shift departments constantly, at the slightest rumor of a reduction in headcount in a particular division), and you spend a great deal of your time where 90% of your employees are waiting for 10% of the team to come up to speed with their job requirements.
Show me a company that embraces the 10% cull, and I'll show you a company that is on the way down the tubes. Companies that terminate the poor performers, not due to some obscure quota, but do to performance - tend to retain their employees for the long haul. IBM used to be famous for this, and they rose to world domination. Motorola used to embrace this, and they used to have a world-class semiconductor market, communications division, automotive parts, space, micro-controllers and cell phone groups. The people make a company great - not the managment. Management has never made a company great, but poor managment has certainly killed more than a few.