As it turns out, both Windows Vista and Windows 7 use the WinSxS folder to point to files that are actually found elsewhere in Windows; in other words, the amount of space that the WinSxS properties sheet says is in use isn't accurate. So, what's the folder for?
By using the WinSxS folder to store what the blog calls the "installation and servicing state" of all system components, Microsoft makes it easier to roll out Vista installations with imaging technology and to patch the image offline (Windows XP and earlier versions aren't image-friendly, and require third-party tools and clunky workarounds to permit image-based deployment). Also, if you get rid of the WinSxS folder, you make it difficult to keep Windows running reliably. So, the word on the street is, "keep the WinSxS folder." To remove old files replaced by Windows Vista SP1, the blog entry provides a link to information about the command-line VSP1CLN.exe tool.