Keywords harken back to academia and the days of the blazingly-fast 1200 kbs modems, providing a bandwidth-efficient way of describing articles and books.
So, naturally, the concept transferred to the web in the early days.
As the web's technology expanded the who basis for keywords dropped; fast and cheap bandwidth let's you be lots less "efficient."
But it didn't mean that keywords suddenly changed into bad things or had a negative effect; it only meant that there were other things, like page title, that played important roles.
People are correct when they say that different search engines place different values on keywords. So my overall conclusion is to use metatags they way they've always been used: it might not help with this or that engine but they can never hurt.
PS: There are an enormous number of different metatags in use that don't seem to have made it into seo discussions. For example, look at the New York Times code (and consider how you might use the other not-well-known tags to stuff keywords without actually stuffing:
<title>G.O.P. Forces New House Vote on Fixes to Health Bill - NYTimes.com</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="description" content="Republicans early on Thursday morning identified parliamentary problems that will require the reconciliation measure to be sent back to the House.">
<meta name="keywords" content="Health Insurance and Managed Care,Law and Legislation,Reform and Reorganization,Republican Party,Senate,House of Representatives">
<meta name="ROBOTS" content="NOARCHIVE">
<meta name="DISPLAYDATE" content="March 25, 2010">
<meta name="hdl" content="G.O.P. Forces New House Vote on Fixes to Health Bill ">
<meta name="byl" content="By DAVID M. HERSZENHORN and ROBERT PEAR">
<meta name="lp" content="Republicans early on Thursday morning identified parliamentary problems that will require the reconciliation measure to be sent back to the House.">
<meta name="cre" content="The New York Times">
<meta name="edt" content="The New York Times on the Web">
<meta name="pdate" content="20100325">
<meta name="ttl" content="">
<meta name="virtloc" content="">
<meta name="des" content="Health Insurance and Managed Care;Law and Legislation;Reform and Reorganization">
<meta name="per" content="">
<meta name="org" content="Republican Party;Senate;House of Representatives">
<meta name="geo" content="">
<meta name="misspelling" content="">
<meta name="dat" content="March 25, 2010">
<meta name="tom" content="News">
<meta name="cat" content="">
<meta name="col" content="">
<meta name="dsk" content="Health / Money & Policy">
<meta name="articleid" content="1247467444861">
<meta name="ARTICLE_TEMPLATE_VERSION" content="700">
<meta name="hdr_img" content="">
<meta name="thumbnail" content="">
<meta name="thumbnail_height" content="">
<meta name="thumbnail_width" content="">
<meta name="xlarge" content="">
<meta name="xlarge_height" content="">
<meta name="xlarge_width" content="">
<meta name="CG" content="health">
<meta name="SCG" content="policy">
<meta name="PT" content="Article">
<meta name="PST" content="News">
<link rel="canonical" href="http://www.blackhatworld.com/blackhat-seo/view-source:http://www.nytimes.com/2010/03/26/health/policy/26health.html">