That is a very good question.
I once read (at Sheldonbrown?) that since the load is shared by the balls, the higher number of balls is better. Retainers increase friction, but make assembly easier and also allow manufacturers to get away with fewer balls. I have seen retainers with huge spaces between balls. So the wisdom was to dump the retainers for better performance, and put in as many balls as will fit in comfortably.
jur is correct. Retainers are mostly about ease of assembly and saving a couple of cents in ball bearings.