Historically, the AO rating has been to videogames what the "DOA" condition has been to emergency room patients: a legitimate status in the world of classification, but one that you never ever want to see applied to anything near and dear to you.
To publishers and developers the dreaded AO rating equates to the Kiss of Death for any videogame hapless enough to fall under its Dark Mark. Most mainstream vendors -- especially image-conscious retail chains such as Wal-Mart -- have strict policies against carrying AO rated games, while the big hardware makers often have codified rules against even supporting games with the AO seal.
The obvious question then is "Why have this rating at all?" It doesn't seem to do anything productive... apart from providing a line in the sand that precious few have ever been willing to officially cross. Publishers will do almost anything to avoid it -- even retooling their content significantly if that's what it takes. The prevalence but uselessness of this 'non'-rating frequently causes the game buying public to refer to the ESRB's classification system as "broken" -- in practice at least, if not in principle.