Adobe AIR 2 Native API/Code

will using Adobe AIR 2 Native API/Code make the app platform specific? if so what might be the use of using AIR in such a case, so that i can have a front end using existing web technologies?

Yep, using native code in an AIR app will make it platform-specific - not only that, but AIR 2 also has another new feature to let you generate native installers (i.e. MyApp.exe instead of MyApp.air), and the new native process APIs are only available if you used a native installer, so the app installer itself is also platform-specific.

The use case is basically that AIR originally blocked access to native processes in order to guarantee that apps would work cross-platform, but lots of people wanted a way to get around this. So they added a way to make platform-specific apps, but only if you purposely use a platform-specific payload.

As for the benefit, you give up the ability to use one installer on all platforms, but you keep anything else that was making AIR valuable to you - being able to use existing web content, being able to develop with web technology instead of C/.net/etc. And of course you still get most of the benefit of being cross-platform, in that most of your app still only needs to be developed once (assuming that the platform-specific part of it is small compared to the rest).