According to Jill Whalen at highrankings.com, "Google May Use The Meta Keywords Tag Again".
Because the keywords meta tags were abused, Google had been disregarding the keywords meta tag. But on the Google Webmaster Blog, Matt Cutts says that it's possible that Google may use the information in the furture.The other interesting thing is that this means that the Meta keyword tag is not completely ignored by Google as most of us have believed. While it's still most definitely ignored in terms of it having any effect on whether your site will show up in a Google search, they do appear to extract the information for other reasons such as this.
There's actually no use for the keyword tag for words that are already appearing on the page. The idea is to use it for keywords that don't already appear on the page, but which might be relevant anyway. After all, if they're already on the page, what good is it to use them again?
Q: Does this mean that Google will always ignore the keywords meta tag?
A: It's possible that Google could use this information in the future, but it's unlikely. Google has ignored the keywords meta tag for years and currently we see no need to change that policy.
I've also noticed that Google uses the meta keywords for adsense targeting. So, it's never been completely true that Google ignores the keyword meta tag.
As I see it the meta tags provides useful information, and it's worthwhile to include on your pages. Who knows?