StageVideo is just a container. It replaces the video object and takes it off the display list. It uses the power of your computer (graphics card) to give better results. It is not just for h264 (although that is where you will see the best results). You can fallback to a standard video object if a user does not have a good enough computer.
I wouldn't worry about stageVideo for maybe 6 months or so. 10.2 won't have enough penetration.
See how the fallback works:
http://www.adobe.com/devnet/flashplayer/articles/stage_video.html