Google’s Matt Cutts posted a YouTube video where he talks about creating tags and categories on blogs for SEO purposes. He pretty much states that this isn’t needed.
“Google is pretty good at saying, ‘You know what? The first time you say a phrase, it’s interesting, and the second time you say a phrase, it’s still a little bit useful,'” said Cutts. “After a while, we sort of realized, ‘okay, you’ve said that phrase, you don’t have to keep repeating it 8, 9, 10 different times.’ So there are certainly some blogs (including some really popular blogs) who have like an entire paragraph full of tags. And they have clearly spent a lot of time, almost as many, you know, minutes writing tags out as they have the actual content of the post. And I always laugh at that because it’s not really that needed.”
He also said that most of the time, tags are words that are already in the post and the tags aren’t necessary.
Google is about giving users the most relevant results to get the best user experience. These types of results aren’t always featured near the top of results, which is considered as an area where Google isn’t really delivering the best results.
Links are only one of the many factors Google takes into consideration for its rankings; the ranks are one of the biggest. The tag pages will show you that links aren’t always the best indicator of relevance.