I don't think many people have images disabled. Images are part of the web for a long time, however fonts are quite new and now every other blogger thinks it's a great idea to have a special, preferably unreadable, font.
But maybe you are right and the image fall-back is not such a great idea. I don't really know how characters are mapped to other font if one is not available but perhaps it would be possible to map them to something better in "standard fonts"? Just guessing here.
