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.