While it would obviously be slimmer if you just wrote
style.css as opposed to
style.css?ver=2009-10-04, there’s a neat little trick to that tail cruft. See modern browsers have rather effective built-in caching protocols which assume that files aren’t changed unless they’re told otherwise. Since the browser reads
style.css?ver=2009-10-04 as two different files, we can effectively let users browser know when we they should fetch our updated CSS files. Which is when we change that tailcruft. So it makes sense to append the date of your most recent CSS file change to that URL, and you’ll be sure visitors see what you’re seeing.
[Update]: Apparently it’s not such a good trick after all. See the comments.