Google, Yahoo, and other major search engines totally disregard keyword META tags and other similar tags, because they're too easy to abuse. (Back when the keyword tag first got invented, porn sites would stuff their pages full of thousands of meta tags in the hopes of turning up on more Google keyword searches.)
The basic lesson here is that search engines can not rely on any user-generated content to determine page rankings, because someone somewhere will game the system if they do.
These days, META tags are kind of a superstition, a holdover from earlier, more innocent days of the Web. Google will pay limited attention to the Description tag, particularly for ecommerce sites (for example, it will pull product names from a Description tag) but totally disregards keyword tags. Google says the presence or absence of any META tags, including the Description tag, has no effect at all on search engine results or ranking, but only on the snippet of text that's shown when a search has already found the site.
