Total employment is a zero sum game (baring structural changes to the overall economy).
Currently economic growth and creation of new jobs is fairly minimal. It is vital to any economy that the employed folks have job liquidity and can move around as needs dictate. Limit job mobility of the employed and the economy will only further founder.
The unemployed are that way for a reason: 1) there are excess people for the current number of jobs that the weak economy can support, 2) those on the sidelines do not have the present skill set to replace a current worker. Barring real economic growth, jobs are zero sum; without the total number of jobs increasing, hiring the unemployed will only turn someone currently with a job into the newly unemployed.