A URL with A-B-C-D-E-F-G equals 2-3-4-5 and a canonical is ABC– why do big sites use this?Adding more canonicals would affect the site.How would Google treat that?
JOHN MEULLER (Webmaster Analyst from Google):
So this kind of goes back into a similar question as before with the canonicals, in that we will notice this is kind of a canonical tag on those URLs. And we’ll try to fold that together. Big sites sometimes do that because any tracking parameters, because they need to do other things within the URL when the user is navigating within a website. So they’ll sometimes add these crufty parameters to the URL. From our point of view, that’s not always ideal. But that’s the way that the web is, and we have to live with that. So the canonical tag is a great way to let us know what you really want to have used as that URL for index.