harddrive123:
Because both are ~5V and your sharing a common ground there, any slight differences in voltage will cause a slightly greater strain on one of the computers regulator when it gets pulled up to the other.
Correct.
Although the strain might not be "slight", it could destroy a PSU.
harddrive123:
Combined with the fact that this happens all the time when you connect any cable from one computer to another that share a Vcc/ground.
Ground is necessary but I doubt they connect their Vcc to the cable.
harddrive123:
Any reason why this wouldn't be true?
Yes, there many reasons why it's a BAD idea.