The immature oocytes are less fertile that the mature oocytes from preovulatory follicles, but the increase in number outweighs the decrease in fertility.  When we only aspirated mature oocytes, we would get one egg 75% of the time.  When we aspirate immature follicles, we averaged 12.6 oocytes from the 451 transvaginal aspirations this year.  This gives us many more chances for a successful pregnancy.