Ah, that's a good point. I back tail-in in my garage so I face this situation all the time. For me, I lean my head over either left or right (depending on which side of the garage I'm backing into) and then I see the "normal" view in the side mirror--with the mirror pointing straight back and the side of the car visible. Parallel parking is another situation where it's a little tougher to use the side mirrors, but just a little head re-positioning should allow you to use the side of the vehicle as a reference.

One other advantage to this mirror position is that it reduces getting blinded by bright headlights from behind. When your side mirrors are pointed straight back and some relatively tall truck/SUV or European sedan with really bright HID headlights is behind you, the reflection is aimed at your face. If your mirrors are pointed more outboard, light from behind reflects away from your face. And bright lights from a car in the next lane generally won't blind you unless their headlights were actually pointed at an angle towards your side mirror, which generally only happens with misaligned headlights.

I was looking for a picture that hopefully makes the geometry of the mirrors more understandable. I found one page just now. If you look at the picture and wait a bit, it will change from showing the side mirrors pointing in different angles.

One commenter on that page makes the astute observation that the mirrors could be swung even wider out. The method I describe involves just barely overlapping the center and side mirror fields of view. But this guy's point is that the side mirrors do not necessarily have to overlap the field of vision with the center mirror at all. You simply need the fields to be close enough that you don't create a blind spot big enough for a vehicle to hide inside. If you did it that way, then you could cover even more area around your vehicle. But I find that many cars don't allow their side mirrors to swing out as far as the "too wide" picture shows anyway so it may not be practical.

The picture and another description of this mirror positioning can be read here:

MostlyHarmless--changing the mirrors certainly does take getting used to. I wouldn't be surprised if you feel disoriented and really hate it right off the bat. It felt really strange in the beginning for me, but now I'll never go back.