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This is a pretty thorough outline of what you need to do: http://moz.com/blog/web-site-migration-guide-tips-for-seos My steps are usually: Identify pages that get significant organic traffic by pulling the Organic Traffic report in Google Analytics for the past year or so. Identify pages that have a significant number of links (or, have links from high traffic sources) in Open Site Explorer. Map where that content should be now, and 301 redirect to new pages. Completely remove all old pages from the index by 404ing them and making sure that no links on new pages point to old pages. Sounds quick and simple, but this definitely takes time. Good luck!

As others have said above, combating scrapers is a big reason, but you're relying on lazy scrapers not removing the tags. Another reason is to prevent rogue content management systems from attaching unnecessary query strings to URLs, creating pages that can end up loading infinite times under different URLs. A canonical tag in the source file would mean that any number of duplicate pages point back to the original. The same reason goes for sites that have issues with redirecting www / non-www URLs to the correct version, or who deal with the same problem regarding secure / insecure URLs. In all these cases, the canonical tag is a bandaid, not a cure - it would be better to fix the underlying problem of the rogue CMS, incorrect redirection, etc. but the canonical tag (self-referencing) is there if you need it. Google doesn't seem to have an issue with it, but if you're at all concerned about the other engines, use with care.

Thank you for all of your feedback. Unfortunately, this website is a on a old propitiatory platform that requires to have these long URL strings, but thankfully there are no separate pages for each of the backslash categories. For now, I have to accept having these long URLs and just make sure all the correct pages are submitted in sitemap. Thank you again for you all of your feedback. This was very helpful!