Difference between Meta Tags and Taxonomy free tagging

SEO advice/articles promoting the use of meta tag keywords as any means of improving search engine rankings are very outdated, usually based on SEO techniques that are no longer relevant, some info dating back as far as the mid/late 90's (for instance Google hasn't used meta keyword tags since waaay back when they started, and Yahoo evidently stopped using them at least 2-3 years ago other than in very very remote circumstances, Bing doesn't use them, etc). If you're reading an article about SEO, make sure you check the date the article was written - if it's older than a few months to a year since it was written, be highly suspicious about whether the info is still relevant... and even then, check multiple sources, particularly the search engines themselves, to see if the advice is accurate. I haven't read the Drupal 6 SE Optimization book, though it may well be parroting old/inaccurate info about meta tag keywords just like many sources do (I was reading that even a FAQ/blog from Microsoft regarding Bing explained/recommended using meta tag keywords in a limited sense, but it turned out the article was copy/pasted from some ancient irrelevant source... when asked directly whether they make use of meta tag keywords for ranking, Bing said "no").

While it "probably" would not hurt to use meta tag keywords, it definitely will not "help" in any of the main search engines. Overusing them or using them in any way a search engine might believe to be "spammer-like-behavior" could hurt you though. The tags might conceivably have some future application for other technologies reading your page, though not search engines. If by chance any search engine uses them at all for ranking, it will be almost no impact compared to the vastly more important aspects of SEO. So, if after every other element of your site is properly tuned for SEO and you feel like adding a brief and accurate set of keywords to pages, I'd say feel free, though it won't improve your rank. Personally I think the time spent adding keywords would be better spent writing even 1 single new piece of content - that one page of unique content will probably bring more traffic to your site than meta keyword tags will for your whole site (if any). Another article I read recently about this pointed out that meta keyword tags are also a great way to reveal the keywords you're targeting to your competitors (they can figure it out using tools to analyze your content's keyword density, but why hand it to them on a platter haha).

Anyhow, the best info is from the search engines themselves:
http://googlewebmastercentral.blogspot.com/2009/09/google-does-not-use-k...

Here's a quote from Yahoo:

What changed with Yahoo’s ranking algorithms is that while we still index the meta keyword tag, the ranking importance given to meta keyword tags receives the lowest ranking signal in our system.

Words that appear in any other part of documents, including the body, title, description, anchor text etc., will take priority in ranking the document – the re-occurrence of these words in the meta keyword tag will not help in boosting the signal for these words. Therefore, keyword stuffing in the keyword tag will not help a page’s recall or ranking, it will actually have less effect than introducing those same words in the body of the document, or any other section.

However, when no other ranking signal is present, unique words that only appear in the meta keyword tag section of documents can still be used to recall these documents.

An SEO's comment I read about this was that if your page's ranking is improved in yahoo due to meta tag keywords, it is a "bad" sign that your page is all but useless - just put some actual content on the page, and the meta keyword tag becomes useless.

Bing has a blog post that seems to promote the inclusion of meta keyword tags as just a tiny fraction of ranking, though when asked in person at a search engine conference, the spokesman said that they do not use it.

Anyhow hope this info helps :)