No matter how good your hiring practices are you will end up hiring some people that just don't work out. If you don't encourage them to look elsewhere, that bottom 10% will just grow (under performers tend to accumulate in safe organizations; while high performers are not afraid to move on to other organizations when it benefits them).
Automotive productivity productive worker to unproductive one tends to be a 2:1 ratio. This balloons to 12:1 for programmers.... in some cases you can even get programmers that are net negative in productivity (they consume more resources than they put back - i.e. code constantly having to be re-written / debugged; corporate overhead).
Not only does hanging onto these underperformers destroy team moral, but it puts all the productive team members at risk since business is a competitive environment.... eventually, your unproductive fat organization will fall to those that are leaner.