Really? Thats non sense... Any particular reason? I used to change the mirrors of my khyber to convex ones.
You can increase the angle of back view by adjusting all the 3 mirrors. Set the center back view mirror such that the rear view is centered. Now tilt the side view mirrors to an extent that you see only a small view shared with the center view mirror. Its not necessary to see the side of your vehicle in the side view mirrors when you are on highways. It is usually done to maneuver in tight spaces and tight slow traffic like pindi traffic or while parking in a tight garage...etc
Plus while driving you should always be aware of the traffic situation at your back, otherwise you will get surprised by the fast coming traffic from behind and could be fatal... or atleast you will get a few angry stares plus beeps...