But it’s worse than that.
As Google tweaks it’s algorithms used for organic search, many are shifting to Pay Per Click advertising as a way to get traffic to their site. When working on Pay Per Click advertising you have to develop a set of keywords and it’s very often that someone will use the same set of keywords on their website as what they are using in their Pay Per Click campaign.
While it’s not necessarily a problem to do this, you are providing a lot of intelligence to your would-be competition if you’re doing it. All your competition has to do is go in and view your source to get an idea of what you’re probably targeting in your Pay Per Click campaign and then go use those keywords in attempt to out bid you.
Since keywords are ignored in the top three search engines in the first place, it’s my thought that you probably shouldn’t add them at all! The traffic you get from the lesser search engines who still make use of this now archaic tag isn’t worth the intelligence you provide your competition.. unless of course you use this to provide them with a bogus set of useless keywords. It’s also worth note to mention that even the meta description is largely ignored and Google only makes use of this if they cannot find adequate page content to create a summary for the search results page.
Ultimately if you’re not using your site as a revenue source or you’re not paying use of a Pay Per Click campaign, then it probably doesn’t matter either way.
Straight from Google’s own Matt Cutts ( Search Quality team in Google )