I can think of one reason that you'd want AIM public, but Skype locked (I know this is the reverse order of the example): Skype has video chat, and is primarily used for video chatting. Aim is primarily used for text-based chatting. There are plenty of people I'll go from typing in comment threads to typing to in real time. The face thing, not so much.

That said, it's not a feature I would personally use, just bringing it up for the sake of argument.