The search engines don’t use keyword meta tags. I think Joost says it is a total waste of time, but there are a few places like some social bookmarking sites that may still pull data from the keyword meta tag. That is a very small reason to use them, but it does only take a few seconds to add them if you are worried about it. If nothing else, it will keep people who don’t know any better from telling you “you need SEO – you don’t even have keywords!”, which is total hogwash.
The reason the search engines stopped using the keyword meta tag years ago is because people abused it by putting dozens of irrelevant keywords in there which made it useless for the search engines. So if you do decide to use the keyword meta tag, make sure you do it right – be sure that they are absolutely relevant to the post (not your favorite keywords for the whole site), and use only a few (no more than maybe 8-10).
If you are in doubt about what is the “right” way to use them, then do not use them at all to be safe.
