Hi!

Hypothetical situation: There was a voltage spike on the +12. Many
drives
use this and it could kill them.

Not usually for the controllers and circuitry. It's usually only the spindle
actuator and head movement assembly (voice coil or stepper motor in old
designs) that use the +12V.

What's it used for on the motherboard?
Mostly the RS-232 which many people would never even miss if you stole it
from their computers.

Quite a bit more than that, actually.

It's available to all the slots on the bus, and I know that some Ethernet
cards use it to power their "media side". At the very least most
motherboards seem to require that it be present in order to power up. Modems
also might require it to be present.

William