I've messed around with it a bit, might try to build something complete with it eventually. It doesn't seem very popular, which means it's harder to find useful examples and explanations to get things working - that's been my problem with it. How would I do XYZ? ... No one seems to know.
I already have Flash, but I've heard good things about Flash Develop as an alternative.
I get the impression it's intended for web developers who would like to create something encapsulated for the desktop. If you're a desktop developer, you probably already have more robust options than AIR. But for the web dev who'd like to apply their skills in a desktop environment, AIR might be appealing.
That's probably why you see more "multimedia applications" - they're common as web apps.