In order to buy music, movies, and apps at the iTunes Store, you need an iTunes account. This is how Apple bills you for purchases and ties your purchases to your computer. So, everyone who wants to use the iTunes Store needs an iTunes account.
However, thanks to a deal between Apple and AOL, you can sign into iTunes using your AOL username and password instead of creating a new iTunes account. This can be very convenient since it means one fewer username/password combination to remember.
So, when looked at that way, you don’t need an iTunes account if you have AOL.
But, think about this: will you be an AOL user forever?
You may someday decide to cancel your AOL account and get a different kind of Internet access (say high-speed access via a cable company). In that situation, you’ll need a separate iTunes account. You probably won’t lose all your purchases in the account using your AOL login, but it will make the switch from AOL to another provider that much harder.
So, given that, and given that an iTunes Store account is free and easy to set up, why not create one, even if you’re an AOL user?