I maintain the SWF Activity Module: https://moodle.org/plugins/view.php?plugin=mod_swf
In Moodle 1.9, I couldn't get the Zend AMF library to work and so used amfPHP ( http://www.silexlabs.org/amfphp/ ) instead. However, something in Moodle 2.x breaks when used with amfPHP; the $USER object fails to instantiate correctly; and I couldn't find a fix. Since then I've been relying on URLVariables; not as powerful and flexible as AMF but at least they work.
Some members of the Moodle community can be decidedly hostile towards ActionScript and Flash which I think tends to drive away would-be adopters. I don't think that the Zend AMF library in Moodle would be missed.
Just my €0.02!