You don't need the mirror to point straight back to eliminate any overlap where something could actually exist though. Especially if you have some situational awareness—a thing can't just materialize right on your bumper; it would have to get there from somewhere, and you should certainly notice if a motorcycle approaches and begins tailgating a few inches off the rear corner of your car. (Any further than that, and you will see them in your mirrors if aligned properly.) For lower speeds where there might be something smaller like a pedestrian in that spot, you can still see straight back in the side mirror with a small movement of your head.
So basically, at high speed, when it's most critical, you get more visual coverage with less motion of your head. At low speed you still have full coverage, with some head motion.
> Especially if you have some situational awareness—a thing can't just materialize right on your bumper; it would have to get there from somewhere, and you should certainly notice if a motorcycle approaches and begins tailgating a few inches off the rear corner of your car.
If you've ever driven in a city with many bicycles and mopeds that is simply impossible to keep track of. No matter how much situational awareness you have, they will seemingly materialize out of nowhere.
> So basically, at high speed, when it's most critical.
I disagree. The most critical is when there is greatest risk for someone to die. That is not if you collide with a car, but hit softer road users (I do not know if it's the proper English term).
City driving, there will be enough to keep track of that some times something will be unexpected. The key is to know your blind spot and check it. At highway speeds, you have plenty of time to orientate you properly with either configuration.
So basically, at high speed, when it's most critical, you get more visual coverage with less motion of your head. At low speed you still have full coverage, with some head motion.