Should You Use It? Sure, For Misspellings
So there you have it — half of the major crawlers (Yahoo & Ask.com) DO support the tag. Should you begin using it? My advice would be only for misspellings and really unusual words.
As explained, the tag can help with retrieval. A word in the tag is treated as if it were a word visible on the page itself. Now that’s handy for misspellings. For example, say you’re writing about Basset hounds. You suspect some people might misspell the name as Bassett hounds, adding an extra T. You could misspell the word yourself on the visible page, but that makes you look bad. You could insert the word and then try to hide it using CSS styles or putting it in the same color as the page background. But this type of “hidden” text is generally against search engine guidelines.
Enter the meta keywords tag. Just do this:
<meta name=”keywords” content=”bassett” />
Now you’ve got the misspelling on your page in a “legal” means that will be read by Yahoo and Ask. You’re still out of luck for Google and Live.com, but two out four ain’t bad.