The USGA says that the most likely score is only if you have not only started play of the hole, but have progressed to a point where a projection is reasonable. It is primarily aimed at providing a score for handicap when playing a match and a stroke or hole is conceded after both players are on or near the green. If you hit OB off the tee, find your ball after your 3rd stroke in the trees with a pitchout to the fairway as your only play, then you concede the hole to your opponent and pick up, the rule requires that you post par plus. I realize that it doesn't sound fair when you would most likely have reached your ESC, but it is done that way to avoid the potential for sandbagging.
I thought the other way too until I saw it direct from the USGA. It's simply an anti-sandbagging measure. It's also thought that such a case shouldn't occur often enough to be a major factor in your handicap.
Got it. I learned something today. I'd have to agree with the last thought as well. I hate I didn't know the rule, but I have almost never invoked the rule I thought it was. I usually play out / finish the round etc.