Comment 114 for bug 1431753

> By design a driver will be enabled if you install it.

However, if it is disable when you UPGRADE the driver, it shouldn't be reenabled.

> We keep an option to disable the driver but [...] That is not meant to be disabled manually.

I don't know about now, but at the time I installed the driver, there was a nice graphical interface, accessible somewhere from the System Settings menu (back when Ubuntu had a menu), where you could choose whether to use NVidia proprietary drivers. And I chose not to.
It wasn't some arcane command line that I found out by googling around, it was something that was made accessible to a "normal user". If things have changed since, you must take into account that there are machines still around that have been upgraded since those times. I can't remember exactly which Ubuntu version that was. It may have been 13.04, or 12.something