Why is it so common? Well, it is the simplist most basic way to get shots on a target. Simply point your plane at what you want to hit, and shoot. Having the other plane turning toward you (and into your guns) makes things that much easier.
As far as being an inefficient way to score kills (50/50 odds at best)- that's actually pretty good odds for a fairly large group out there. Anyone who has less than a 1/1 K/D is doing worse than 50/50...
Why would someone not take the HO shot? An "ace" might avoid the HO for several reasons- 50/50 odds aren't good enough, and he/she'd rather maneuver for a safer position to shoot from, or maybe he/she sees the HO as a "low-skill, beneath my level of expertise" tactic. For someone not all that skilled, those ideas don't really apply. They may just want the kill, and not care about how they get it. And honestly, against a superior pilot the HO may be the best option they have to win that particular fight. If they try to out-maneuver the ace their chances of winning the engagement may drop significantly below 50/50. For them the option may not be about whether or not to take the HO, but whether or not they can get the shot off before you do (whether you intended to shoot or not...)
In reality, it's a no-brainer that if you let your opponent get his guns on you he's likely to shoot at you. If you're getting hit, you've screwed up. Who really cares what angle the bullets come from? I think the real reason the HO is such a sore subject is that folks are embarassed by being beaten by the simplist, most basic shot possible, before they're able to put their super-uber maneuvering skills to practice.
The trick is to kill the other guy without getting killed yourself- not to kill the other guy because he was nice enough to not kill you when he had the chance.
