I would also argue on this one. The biggest reason this happens and is automatic on every post if for when people (and bots) scrap and republish your content. 99% of the time people will copy the rel canonical tags to their site and post. This helps Google say, these guys are still attributing the content to your site.
I write about this stuff for Forbes and on my personal site. Let me know if you need me to go into more detail.
