Some people may prefer attack speed and it also works great with Irelia's (W). I just don't think it's necassery, because I will buy an item with attack speed like triforce wits end (ionic spark/statikk shiv) and I prefer the flat AD which helps great with last hitting and early pressure.
After Pre-patch-s3 is launched i've concluded:
21/9/0 is not efficient and will make you a lot weaker than you gain with damage I would suggest you to go 9/21/0 because the extra sustain/tankyness will help you early in the game when you are weak, and you will also benefit a lot more. Especially the reduced cc and more resistance.
When laning against AP take 3 points in resistance instead of hardiness.