I am currently using a CMS that is not optimized for search engine friendly websites. This CMS gives me the possiblity to define global keywords, so that they will appear on each site page.
Imagine that my website is about sport (it is not), and that I have articles about soccer, rugby, swimming and so on. Does it make sense to put as keywords swimming, soccer, rugby etc knowing that in this way I will have the "rugby" keyword also on all soccer articles? Or would you just define at this point very few generic keywords like sport, sport news, etc so that they might be present sitewide?
The meta keywords tag is almost universally ignored by today's search engines. But if you're going to use it (and I still do) the keywords should NOT be universal but specific to each page. I put in the important keywords I hope the page will rank for as a kind of "memo" for the future.
Yahoo does look at the tag (at lest they used to) although it has little influence in the algo. Their recommendation, as I recall, was to use it for alternate spellings.