I've had limited exposure to self-defense classes, but I have qualified several times with a handgun for the military**.
In those cases, we were taught a similar method of draw (Strong side to chest and then out to target).
When I was taught, I was told that one of the primary reasons we were learning to draw like that was that it actually provides three areas to shoot from.
As soon as it clears the holster, the barrel (and your arm) are turned parallel to the ground. If the BG is closing in, you can shoot from that position with relative stability (maybe not awesome accuracy without practice, but relative stability).
When you bring the weapon to your chest, now supported by your off-hand, you can still shoot from that position. Maybe not comfortably, but you know what I'm getting at.
Then, if the target is still a "safe" enough distance, you bring the weapon out into a good isosceles stance.
As far as the one-handed shooting, I do that routinely for practice, both strong- and off- hand, for some of the reasons stated above. God forbid I have someone with me that I need to shield, or I've been injured myself, I'm holding off another BG, etc.
**I'm a reservist, so I dont get nearly the training with a handgun I'm sure the active guys do/did, so hopefully someone else can chime in. I do alot of training on my own with my own handguns and AR-15, so I'm sure not everything I do is "proper", as it were. Personally I wish the reservists would do much more shooting that we currently do...