The problem with that method is that it requires that you be 100% confident in the placement of your mirrors because there is no feedback on their location until it becomes critical.
So did the old method, until people got used to it. This method, however, works better.
In contrast, when the mirrors are showing you part of your car, it provides an automatic reference point to judge the location of the images that does not depend on the precise mirror position. Furthermore, if your car does not show in your side-view mirrors, then it indicates that their alignment is off.
Since the standard interface with rear and side view mirrors includes easy adjustments, I'd rather not depend on the placement being accurate.
I'd rather have a proper view beside and behind my car without having to physically move my head (I still do anyways, but it gives me next to no new information when making a maneuver).
As to the notion that people will move your mirrors around, many new cars today come with memory seats and mirrors so you can have settings for 2 or 3 drivers programmed. Simple.
I, on the other hand, work in the car industry so I can set my seat, wheel, mirrors (and optionally pedals) and radio station presets in any given car within about 45 seconds, so it's not a huge problem for me. However if you practice in whatever car you personally operate you can get the hang of it in a few hours of driving.