Fixed and this makes 100% perfect sense.
If you no longer have a valid email address connected to the IM ID you should NOT be able to use it.
Right now, as it stands, it's technically a bug. If an email is no longer valid, the ID shoudn't register and should not be usable. That just makes sense.
MobileMe is fixing this. I see this as the 'correct' way to do this rather than how it is now.
If you want to use the phone analogy, it's like getting a phone, canceling the service and you expect to continue to get phone calls. Is that right? NO. For that matter if you don't renew, I would hope the name gets discarded so someone else can pick it up. Same with phone numbers. If you cancel, then you've pretty much put your phone number back in the pool of numbers.
This is more a fix rather than a 'direction' they are going.
You can still use an AIM ID instead of a .Mac or MobileMe account and still have access to most if not all the iChat features. Besides aren't AIM id's and .Mac id's are different aren't they? I know people that use iChat with AIM's that don't have .Mac. MobileMe has nothing to do with current AIM accounts. However, if you use a mac.com email in your AOL profile I would suggest changing that ANYWAY since it poses issues in itself.