AFAIK Manchester Oxford Road is actually the busiest of the 5 Manchester city centre stations for peak-time commuters (or it might be second busiest after Piccadilly?)
Certainly, in the days of my tortuous commutes into Manchester (from Atherton line), there were usually FAR more people getting off trains at Oxford Road than at Piccadilly, (and far more people getting off at Salford Central than at Victoria).
I guess people travelling eastbound into Manchester that work, say equidistant from Oxford Road and Piccadilly will get off at the first station their train calls at, which, in that case, would be Oxford Road.
Off-peak usage of Oxford Road tends to be fairly light, and there could be a school of thought for stopping fewer trains there, in particular, the Manchester Airport - Scotland services, which, when operated by Virgin, never did stop at Oxford Road. But the linespeeds and sheer numbers of trains passing through this section would almost certainly see trains facing red signals at some point, so very little, if any, time would be saved by not calling.
In some ways, Deansgate is a better place for trains to call, because there is direct access to / from Metrolink, but station facilities do not lend themselves to dealing with large numbers of people. That said, there are at least lifts at Deansgate, whereas at Oxford Road, people using platforms 1-3 are still forced to struggle up and down stairs.