Okay, the fact that you haven't heard of the 21 Foot Rule probably explains a lot behind your reasoning. Basically, 21 feet is about as close as you can let an attacker get without him being so close that he could charge and attack you with a melee weapon before you can draw your gun. Watch
this video and you'll see exactly what I'm talking about. It's a much longer distance than you'd expect, and not everyone is a quick draw artist. It gets even worse if the attacker was already running by the time you noticed him, and it's unlikely that even 30 feet would give you enough distance to draw and shoot. Trying to draw on a guy with a knife and fire a warning shot when he's moving toward you is downright suicidal if he's less than 20 feet away.
On the subject of warning shots into backstops, it's impossible to truly be able to compensate for what's behind your target every time you fire. For one, the target may be blocking a far away person or object you don't want to shoot. But it's quite often that you won't be able to immediately adjust your position or point of aim in a self-defense situation to ensure that there's a safe backstop behind the target. I'm willing to bet that if you were placed in a real CCW test situation and asked to fire a warning shot off the cuff without prior preparation, you'd hit or come dangerously close to something you don't want to shoot unless you were out in an open field.
Again, if the situation has not escalated enough that you're actually ready to shoot and kill someone, don't pull out a gun and try to fire it in a non-lethal manner. The same goes for trying to shoot an attacker's limbs. Or worse, trying to shoot a fleeing suspect in the legs and hoping that it simply disables him and you didn't plug his femoral artery with a bullet.
