Why do you say that? iMessages supports AIM accounts so if their friend on Windows want to talk to the iMessages user, the iMessages user just needs to add the AOL account in their accounts in iMessages.
So tell me why NOT.
OK. The reason why many people (like me) use iMessage is that it integrates traditional SMS and "enhanced" instant messaging into a single unified inbox on every device (Macs, iPhones, iPads).
Adding AIM into the mix breaks the unified inbox model. AIM isn't supported by iOS Messages app, it requires a separate app and a separate account.
To give you a specific example.. When I send a message to my wife - iMessage delivers it to all her devices. She gets my message on her iPhone (when she's on the go), and her PC (if she is working on her Mac). She can choose either device to respond from, and it's very convenient and seamless.
Replace her Mac with a Windows PC in the above scenario. Now my iMessage to her is delivered to her iPhone only, but not to her Windows PC. Conversely, if she adds an AIM account to her Windows PC - I now have to use AIM to send her a message rather than iMessage. And she has to install an AIM client on her iPhone to get that message on her mobile device. I may as well stop using iMessage altogether, and switch to AIM (which none of my other friends use). That is why AIM idea doesn't work.
If Windows can support native iMessage - none of this is an issue.