If you order str8 run, the males will be about 10 days ahead the females in weight. In other words, if you butcher a 7 week old cockeral, ten days from then, the female should reach the same weight. It's nice if you are planning on butchering them all at the time and want a variety of sizes. It's not so good for me because I want larger birds that I can butcher all at the same time.
First reason for not letting them go longer is their feed to meat ratio goes to hell once they hit that size/age. Actually, it starts to go downhill after 6 weeks, but I like a bit larger bird, so I accept a little less efficiency. Secondly, once they get a bit larger, there's no guarantee you won't find them on their back dead the next day. I know there are people out there who raise them past 8 weeks and have no problems. I just don't feel it's worth the risk of losing a bird at that stage in the game, especially since I have about $8 invested in that bird, plus the $12 profit I stand to lose if I am selling it.
Hope this helps!