Although a month old, for those who didn't pick up on this one (me included) this article highlights two serious vulnerabilities to Windows Defender: http://www.zdnet.com/article/windows-10-uks-gchq-found-out-how-to-hack-windows-defender-to-own-your-pc/

They should now be patched but for me it highlights two important points. One, don't turn off updates or you won't get the security patches to fix issues like this. Two, don't trust Windows Defender - there's a reason you have to pay for the best antivirus products!

Everyone's experience is different. I can't imagine turning off updates. I know many folks do. I've never had a problem with one. I also can't imagine paying for AV. None are perfect. I've had to fix many computers where pay AV was installed. It slowed down the computer ( seen as a trade off for protection) and didn't work anyway. Maybe those people are a minority, but I hear stories that they weren't happy to pay for AV then have to pay some tech person at the store a couple hundred bucks to get their computers working. 

Heck, just turning your computer on can be dangerous.