as per ACTM, duties of traction operating offers involves the following
:
c) Maintain close liaison with the Operating Department officials and keeping himself fully conversant with operating problems and evolving counter measures to get over them so far as electric stock is concerned. Ensure maximum utilization of locos by watching detentions, taking on additional services, improving the engine links etc.;
.
There...
more... is a clear mention of 'improving' engine links, so it is clear that zones regard certain class of locos above others. Of course, stopping to use an 80 kmph derated loco for passenger operation and instead going for WAG5P is also an improvement, but even within passenger locos I believe they don't treat all locos equally.