Despite what some people think, *every* image should have an alt text. These are
the W3C specs (http://www.w3.org/TR/WCAG20/#text-equiv), aimed at making the web
more accessible for people using non-visual browsers, text-based browsers, or
browsing with images disabled. Wikipedia is supposed to be an encyclopedia for
everyone - including the blind, the colour-blind, and people with limited
connections and devices.
Either the markup for including an image in a page should require alt-text, or
alt text could be made compulsary on the image page, and automatically included
in all pages using the image.
This is a MediaWiki issue, because making alt text compulsary is the only way to
make MediaWiki pages validate for basic accessibility.
For a fuller explanation, see
images should have alt text . Please keep comments here technical rather than
idealogical; otherwise comment at Wikipedia.