Also, since workers are not supported on mobile AIR platforms
, you wouldn't want the framework tied to them. Otherwise, you'd write an app for web and desktop but then not be able to use it on mobile. While UI's usually change between these platforms, business and domain logic is generally reusable if best practices are followed.
1) Worker AS3 Doc http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/system/Worker.html
"Note: The use of workers for concurrency is only supported in Flash Player and in AIR on desktop platforms. Concurrency is not supported on mobile AIR platforms. You can use the static isSupported property to check whether concurrency is supported before attempting to use it."