I've found that since many owners don't utilize the Rookie league much, it is retardedly easy to make the playoffs. So putting your good first year prospects there gives them a great chance to get in extra games-- get through a few playoff rounds and your guys will get a 1 or 2 point bump in many categories that may have been unattainable at upper levels where they might not have made the playoffs. If you have halfway decent coaches at the Rookie level, it's like a ratings bump gift.