Did I say anything about Flash? I was talking about a video tag. And are you saying that a 4 seconds video in the video tag will be less efficient than an img tag with an animated GIF?
Personally I see no point in using GIFs, unless on a site which allows embedding images, but not videos. On a site which you control, video tag works just fine.
- ignore older browsers (today practically all browsers support the video tag, caring about older browsers in this context is like caring about IE6, i.e. it's bad).
- stick with a compromise solution (H.264, yeah that's sad, but until Daala will come this mess will remain, but now Firefox already supports H.264 easily, so it's not a problem anymore). If the space allows it, I wouldn't mind encoding in both WebM and MP4, just for the sake of supporting free formats.
I'm guessing for legacy reasons. They've been using gifs for a long time. If this was the first time they needed an animated video, perhaps they'd jump to video tag.