Why make it play go with your site when you can make it play checkers?
Nobody's claiming Google can't figure out the visual structure of a page or determine the canonical page within a set of duplicates, but evidence[0][1] has shown that making it easier for Google to do those things has a positive effect.
[1] Anecdotal evidence: Just two weeks ago one of my clients' sites had a temporary issue with 301-redirects, causing some old URLs to return 404 instead of redirecting to new URLs, and the www version of every page not redirect to the non-www version. Search traffic nearly halved the following week (this past week). I patched up those issues and we're already seeing things resume back to normal.
Nobody's claiming Google can't figure out the visual structure of a page or determine the canonical page within a set of duplicates, but evidence[0][1] has shown that making it easier for Google to do those things has a positive effect.
[0] Search Engine Ranking Factors 2015: https://moz.com/search-ranking-factors
[1] Anecdotal evidence: Just two weeks ago one of my clients' sites had a temporary issue with 301-redirects, causing some old URLs to return 404 instead of redirecting to new URLs, and the www version of every page not redirect to the non-www version. Search traffic nearly halved the following week (this past week). I patched up those issues and we're already seeing things resume back to normal.