Because, Windows may have important security updates, which if you miss to install, may leave your system vulnerable and prone to malicious attacks.
It's been a year since I disabled the update until now it doesn't have a critical problem about it, as long as the Rig is only installed, what is needed for mining (claymore, GPUZ, atikmdag) is likely not to happen.
but for this, I agree
It's better to set them on Manual.
When you do Sh*t things other than mining, you might need an update and so is true to your AV's
not against of it, however some major updates really F*cking up drivers
its a main goal to keep mining without interruptions and if rigs is only dedicated to mining then no worries