
Do you trust windows to download all of the appropriate updates enough to turn on automatic updates and leave it unattended to monitor your machine? Would you rather take the time to check for updates that you may need periodically?
Personally It depends highly on the machine in question for me. My internet work horse of a machine I leave automatic updates turned on so that I generally get any new security patches as soon as they are released. My company machines and servers I prefer to do with my attention on the update as it installs.
On my home pc, I enable automatic updates, but at work, we disable it because not all windows updates are 'good', and need to be tested in our environment first. This is probably the one automated feature of Microsoft's that I actually trust because it hasn't failed me yet.
On my home PC it's disabled, because I have an older machine. My office is set by the Network IT guys, so I have no choice but to have it enabled. Still, I make sure to check periodically.
This is a perfect example:
Windows patch problems to force out-of-cycle repair
Critical updates are automatically download with automatic updates, this patch was out for nearly two weeks and Microsoft find a problem with it and releases a second patch to correct the problem.
*For this reason, if I depend highly on a machine I do not trust Microsoft to decide what is best for what I have running and what I am using the machine for.