Still, I applaud the hacking :)
You're right, you're not going to use this to encode movies or very large media files, but for encoding shorter clips, the performance isn't unreasonable.
Encoding videos on the desktop with the native FFmpeg code will not only yield better performance, but I can't think of a scenario why you'd want to use as a web-based service.
Create screenshots? Really? What a complete triviality. I guess using standard software and hitting the PrtSc key is too difficult in comparison to using the browser for everything. Making screencasts is a different issue, but also one you'd do locally.
Sorry if I'm not so passionate.
That list changes every year, too. As JS runtimes get faster, more and more is now possible in the browser that would've been fully impractical just a few years ago. In just a few more years, I predict that stuff like video processing won't be so far-fetched either.
(shameless plug) more editing tools coming soon :)
Funnily enough I'm using a color pallete plugin written by @bgrins who is working on the OP project.