I voted believable because I like, or don't dislike, the mechanic. As to the believability, meh.
I think a lot of this comes down to how fast and loose you like the HP abstraction to be, and perhaps that's reflective of how you like your games to feel. I like my (fantasy) games to feel fantastic, whimsical, magical, heroic, et. al.. I like my HP abstraction really abstract (possibly as a result). I don't really care for gritty games, but I imagine there's a correlation between the people who do and the people who want HP abstraction less abstract.
For the games I like, I can rationalize just about anything narratively and use abstract HP as a simple, representative mechanic. Level 1 wizard mauled to 0 HP by a housecat? Maybe he didn't really die but was so shamed by the experience that he returned to his library to study and tell wild tales about how he used to be an adventurer (until he took a tabby to the face).
Damage on a miss? Maybe that's bruises from dented armor, maybe it's "luck" running out, or maybe it was just such a near miss that the foe had a serious sphincter pucker moment, weakening his resolve and ultimately reducing the amount of fight he's got left in him.
If my real life becomes more fantastical (say, a dragon attacks my village), I'll probably rethink my stance on HP abstraction. Until then, the whole game (as I like to play it) is an abstraction, so I don't mind abstractions within my abstractions. Abstractception!