Whether Google uses meta keywords tag or not, has always been a confusion. However, Google's Vanessa Fox recommends using meta description tag for each page.
Vanessa in response to indexing issue at Google Groups wrote:
"Looking at your site in the search results, it appears that your pages would be well served by meta description tags. For most queries, the generated snippet is based on where the query terms are found on the page, and in those cases, your
results are fine. But for some more generic queries, where a logical snippet isn't found in the text, the generated snippet seems to be coming from the first bits of text from the page — in this case, boilerplate navigation that is the same for every page."
According to Vanessa, a meta description tag for each page facilitates Google to determine the uniqueness of the page. Otherwise, Google uses first few words appearing at the top of the page.