True, but why not make the experience better for browsers that support these standards and have a decent, albeit not identical, fallback for IE?
to do otherwise just adds more work, not only for detecting the browser, but for making sure the page layout and so on works in both versions.
Personally, my strategy is simply to not write HTML or CSS. I write to a DSL that is implemented based on what browser is being served, so there are no sniffer scripts in the pages themselves. True, this does mean the same, otherwise static pages, may need to get generated over and over, but this is where caching comes in.