That's precisely why it would be interesting. Getting cleaner decodes of jpeg-encoded line art and screenshots would be great.

Some images are only available as badly encoded jpegs.

Fonts suffer from ringing, their lines can actually look better if they are on the block boundaries.

Yes but there still isn't a universally supported "good format" so JPEG often gets used for text.

Because it might still help if someone else made the wrong choice in image format.