Kevin Trye on "[Plugin: WordPress SEO by Yoast] keywords are not important for SEO?"

Meta keywords have been ignored by google and most other search engines since 2004/5, hence it's turned off by default by yoast. Google looks at page titles and content instead. Just because others do it doesn't make it right. Often html software will still generate the tag entry in the header, but there's little point in adding anything to it. It's likely only useful if you setup a site as part of an internal intranet where it could be handy for local [company] searches.
Ref http://tinyurl.com/839ya7w