Around a year ago, Danny Sullivan did some great research on the Meta Keywords tag  to determine which search engines are using it. He found indication that Google and Microsoft Live Search ignore it for keyword ranking (retrieval) purposes, while Yahoo! and Ask are apparently using it. With Google having the bulk of the search marketshare, and Yahoo possibly only using the tag to a limited degree, it would seem rather extraneous to continue using it for search optimization.
Although his research was really pretty definitive in my mind, there are so many search marketers that have some sort of nostalgic devotion to the tag and who continue to obsess over it and insist upon using it. There is a sort of mentality that “if it might help, then I’m damn well going to use it.” (See this recent thread  at WebMasterWorld where quite a few express this viewpoint.)
However, I see some compelling reasons to avoid using it altogether…
Yahoo may be using it only as a “signal of last resort” — if they can’t find content matching a keyword search through other, preferable signals such as visible page body text, they might only then fall back on meta keyword content. As such, the keywords tag may not provide any sort of additional ranking weight, but may only be worthwhile when no other pages or visible text content matches terms. In this case, if you already have the terms in the visible text of the page, it’s just not necessary to have it in the Meta Keywords tag – your page likely won’t rank any better than it already does.
Google may still be using the content as I’ve outlined previously (see this article on the Resurrection of the Keyword Meta Tag ), though not as a keyword ranking factor. It may be used by them as a negative ranking factor used to assess a page’s quality or to detect spam and “over-optimized” pages. For instance, they may be checking the tag to insure it represents text actually found on the page and that it’s not too crammed-up with terms. Pages with unrelated words in the meta keyword content or which are stuffed too much might be singled out for lower quality scores or penalizations.
If I’m correct, then the meta keywords tag can’t really help your natural search optimization, but it could easily hurt it. I’ve seen so many cases where people have placed all identical meta keywords tags throughout their site, or included words not found on the page, or over-stuffed the tags — since so many people do this wrong, it’s better just to leave it off entirely. It’s just not worth it.