Check out this web page.
http://www.bardenbearings.co.uk/index.cfm?articleid=635
Basically it says the load carrying abililty is greater without the retainer but "With the retainer removed from the bearing the balls are no longer spaced apart and can make contact with each other. With the inner ring acting as a sun gear each ball is driven in the same clockwise direction. At the points of contact the balls are travelling in opposing directions leading to friction and wear. Due to the introduction of ball to ball friction the bearing torque is also increased. The use of ceramic balls can provide lower wear levels and reduced bearing friction/torque."