Why do some engineers prefer minor rusting(rust stain) of rebars in RCC works? Will this effect the strength of RCC structure?

Minor rust on steel will not affect the strength of steel because the steel size is not reduced unlike for totally rusted and flaking reinforcement bars that its size is lessened so definitely the strength is also lessened. So if minor rusted reinforment and severe rusted reinforment are incorporated to concrete to form RCC, concrete with severe rusted reinforcement have more chances of failure for overloads or during earthquakes.

rusting roughens the steel surface and provides a better bond with concrete and a minor rust surface would occur anyway inside concrete caused by concrete's water. Anyway minor rust is hard to avoid- you have to store the bars somewhere before use and how can you find a place without oxygen and moisture? Rusting is not a disease to be spread, so the minor rusting is not followed by extensive rusting especially in the protective concrete environment. On the other hand, rust was steel before, I mean it weakens the steel by definition- and how safe is it to judge that the rusting is minor? 

minor rusting may not get effected for rebars that to minor rust gives the good bonding between the reinforcement and the concrete.