@tkramar raised the question on Twitter, why every desktop client for that service seems to be built on top of Adobe Air (here). That’s something, I also was wondering about. And in fact you often hear people complain: “Why AIR? Why not a native application?” Let me try to give the answer while having breakfast. 
First we need to look at what Adobe Air is: It is a run-time environment, ported to all common operating systems, which allows to run applications built with Adobe Flex on almost any desktop rather than embedded with a browser (where you can run the same applications with little or no modifications using the well-known Flash Player Plugin).
So, we know that AIR developers use Flex. Why is that? That’s as easy to answer: Flex targets front-end/GUI designers in the first place. Together with Flex, Actionscript, and Flash, you can build quite neat and shiny user interfaces. Most of the “business logic” (I know that sounds funny in this context) is not part of it though. The strength of Flex is to use external data sources like web services. On the one hand, it’s certainly a limitation, but on the other hand that’s exactly why it’s being used for Twitter clients, which don’t do much more than using Twitter’s API (web service) to display tweets, subsets of tweets, search results, friends and stalkers (erm followers, of course).
So if you want to bring a desktop application to the market, which heavily depends on web services, Flex is probably one of the fastest ways to get there. And Adobe AIR brings the application to the desktop — on all common operating systems.
Yes, it’s proprietary software, and closed source. However, anyone can use it free of charge (the Flex SDK is free, the not required Adobe Flex Builder is not). Pretty similar to PDF and Flash. But the licensing policy is another discussion, anyway.
