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Our site currently has some keywords that do not appear elsewhere in our content. The reason is that we believe some people will search for us with certain keyphrases, even though we do not use those particular phrases to describe ourselves.
I have been inclined to include more keywords to cover more search possibilities, but I won't if doing so will bump us down further down the list on searches of our most important keywords.
I mostly have google in mind with this question. Let me know if I should post in that forum instead.
Not too long ago, AltaVista was encouraging the use of synonyms and alternate spellings in the tag, and other engines were saying that they took a dim view of including any word there that isn't on the page. Now the whole issue is just one big yawn - only a red herring and a distraction from the important factors that make a good, well optimized website.
Search engines can't afford to put any significant weight on an element that's hidden from visitors and almost totally open to manipulation. So it's best to have no concern about the tag. There are top ranking pages on competitive searches that have no meta keywords tag at all. You don't need it, unless you have a site search that uses it.
As I mentioned in another thread, I use the meta keywords tag as a convenient place to keep notes about what I was optimizing that page for - but nothing more than that.