Been doing this for a decade and it really works with my brand of assault er defensive driving. Only shortcoming is if you don't have a rear view you have to revert to conventional settings. If you are in a multilane environment there is a possibility that an overtaking vehicle can still start in a blindspot 2 lanes away from you (4 or 7 o'clock) and then move into the same middle lane as you are going for. Exit ramps introduce similar possibilities, so look!
Done properly with a continuous scan of all 3 mirrors you know where everyone is on the road, even better if you maintain a steady and slightly overtaking speed of most traffic. Dramatically reduces night fatigue from headlights shining in your sideviews too.
I've done it that way for more decades than I can remember. That's why they call 'em "side" view mirrors -- so you view what's be"side" you.
My parents were from the generation when automobiles didn't even have mirrors on the sides, only a mirror in the middle of the windoshield frame to see who/what was behind you. When exterior mirrors started becoming available as options, my parents bought them, and referred to them (actually, it is "it," since in those days they were only on the driver's side) as "the outside rear view mirror." And that's how they used them (it) -- adjusted to look behind the vehicle, duplicating the job of the
inside rear view mirror.
Once I started driving, it didn't take long to figure out that there wasn't a lot of benefit to having two (or three) mirrors looking at the same part of the road, and that there was a lot of benefit to using those "outside rear view mirrors" to scan the blind spots off the rear quarters. So that's the way I do it, and it works. I've never seen any need for those dinky little "blind spot" mirrors. They're all so small that anything smaller than a 747 is almost invisible, anyway.
