Apparently, the cost of applying this critical security fix is a lack of system stability or even a dead machine on certain installations.
"Some users contacted by eWeek last week said that when they installed the patch on machines running Windows 98 and IE 5.5, the PCs failed to reboot and instead returned a "scandisk" error message. When the users tried to bring the machine up in safe mode, it froze, preventing further action."
This isn't the first time Microsoft has done something like this. One of the first versions of Windows Media Player 7, which Microsoft was desperate to get out, contained a CD-writing plugin from Adaptec that conflicted with Adaptec's own Easy CD Creator Software. The result was a toasted installation that never booted again. At least, not in anything other than safe mode and as I recall even uninstalling the software didn't help, forcing the user to restore from a backup or reinstall.
Microsoft deny that there has been any such widespread problems but this article here from Eweek.com would seem to disagree.