Being vertical actually makes it harder to control your position in the water (all 3 dimensions) and it's more than likely that at least one diver in an OOG situation will be having bouyancy issues. If you continually practice OOG being horizontal and neutrally bouyant (thus in control) then it is more likely that the divers won't make a real OOG any worse by bobbing up and down in the water column.
What he said.
To elaborate, being horizontal means you have much more surface area resisting changes in depth, which makes you more stable. Plus your most powerful tool-of-last-resort for controlling a rapid ascent is to kick down, which is difficult to do if you are vertical in the typical head-up position.