It's not MUCH security, I admit, but some folks like it as one more speed bump. Essentially, it makes sure the secure log-in page is up so you're not accidentally revealing your username and password to malicious software.
Here's how to put it back into action in Windows 7.
Go to the old Start button, now just a Windows logo. In the search box, type netplwiz and press enter or click on the result.
A pane called user accounts will appear.
Click on the Advanced tab.
At the bottom of the screen, under secure log-on, check "require users to press Ctrl + Alt + Delete"
Now when you log-in to your computer, you'll have a rather boring pane asking for Ctrl-Alt-Del, which you'll have to press before you can get to the log-in page.
Hope that makes you feel just a little bit more secure.