How's this for impressive?: http://www.jamesward.com/2007/12/12/blazebench-why-you-want-amf-and-blazeds/
Currently, at least 30% of web browsers don't support HTML5, whereas at least 95% have Flash Player installed. I think Silverlight's currently at around 70% and it wouldn't be unreasonable to ask learners/IT depts. to install the plugin.
Thanks to Adobe's Open Screen Project, AS3 and Flash are open specification and open source: http://www.openscreenproject.org/
Then, the Flex Framework makes developing RIAs easier than ever, as do open source projects like the Open Source Media Framework and the countless AS3 library projects available on GitHub and Google Code.
Or more specifically in Moodle, you already have a framework for integrating Flash with Moodle 1.9's back-end infrastructure: http://code.google.com/p/swf-activity-module/