There are situations where a canonical tag will help sort out URL->content confusion, as discussed very thoroughly in the recent homepage thread.
Dynamic pages with unique URLs that order or sort or highlight or annotate page contents in subtle ways may have very similar - or identical - content. Instead of plopping a "no-index" on those views, or employing a "scrub-and-redirect" technique, you can put this tag on the page and it's much simpler, sematically honest, and possibly more effective. Actually its effectiveness remains to be proven; I hope in the next 6mo someone here will bless us with some empirical evidence to show this tag used successfully to its fullest potential.
A very laisser-faire attitude, a luxury enjoyed by those whose sites are naturally popular, enjoy other traffic streams, or don't NEED the leg-up from good SERP placement. For sites that only stay viable on the merit of their good SERPs, understanding this new tag is significant, just like understanding <meta> tags was significant 10 years ago.