But you need a radioactive rating. There are games that almost all parents don't want their kids playing, and they assume that they're going to be found under the most restrictive rating.
If you get rid of AO, you provide Jack Thompson and those like him a great deal of ammunition. It would be perfectly accurate to say that the "ESRB doesn't think that Halo 3 is any more appropriate for your children than some sort of hypothetical rape simulator".
The whole point of AO is to be a beyond the pale rating that no retailer wants to carry. This allows them to market less vile games under the M rating without the same stigma.
Edit: Basically, you strip the industry of the ability to argue that it can effectively police itself at all. Many parent will let their teens play Halo, but many wouldn't want them to touch Manhunt. By failing to rate the two games differently, the ESRB fails to adequately police itself in the eyes of many parents.
