In the previous posts, I've mentioned the importance of having a clean and crawlable website structure for SEO.  The reason is simple.  Without one, you're dead in the water from an SEO standpoint.  If your content cannot be crawled and indexed, then you can forget about the other aspects of SEO, including content optimization, linkbuilding, etc.  As part of my SEO technical audits, it's usually not long before the important topic of canonicalization comes up.