If the motherboard is hooked to an ungrounded case it is as good as being ungrounded when a human comes along who is in contact with earth ground, or any other ground for that matter.

"In that situation the case itself serves as ground because it is the common reference point for the electronics", "The circuit doesn't care if it's earthed."

And when you come along grounded to earth that common reference point is no longer common. It cares when an uncommon ground comes along that is earthed. The earth ground will be the on that takes the build up.

"Do you think the space station can't have any electronic working on board because NASA forgot to run a ground wire up from the planet?"

Read my final edited in option to cover situations where there is no earth ground. Both objects (you and the electronics) need to be with common ground, not one grounded to earth and the other grounded elsewhere.

This is a basic grade school science experiment. It doesn't take a degree to understand.
A line of people hold hands (common ground circuit). One on the end builds up static. Someone comes to the opposite end (non common ground) and gets zapped. The ones in between the electric maker and the new ground did not get zapped because they were on common ground.

If you want to take what you remember from getting your degree and put it in practice. Cut the ground prong off an appliance, open it and put a hot wire against it's case. Then tell me that an electronic items case is the only ground it needs when you add a human ground to the equation. Do you think that no discharge will pass between you in that situation because the case is the ground for the appliance?