I can hear the technical objections now: "But it wouldn't *do* anything. Nothing whatsoever has happened in the Mediawiki database until you hit 'submit', so there's nothing to cancel. The right way to cancel is to hit the browser's 'Back' button, or to click on the mediawiki 'project page' or 'discussion' tab at the top (as appropriate), or to close the window. So it turns out that having 'cancel' be a link is actually The Right Thing."
(None of which is actually a valid argument against the idea from a user interface perspective, of course. In fact, it sounds like a perfectly fine idea to me, even though I'd never personally use it.)