You'd have no chance doing all of Hyde Roads driver swaps in the city. Any city where driver swaps are in the city/town, it tends to lead to the driving coming off dumping the bus and then wandering off for their break and then the relieving driving not knowing where the bus is parked or trekking half way across the town just to find the bus (as happens in Burton on Trent with Diamond). Manchester City Centre already has a lack of layover space for buses, having buses sitting around the city centre to wait for relieving drivers could make the city just a dumping ground of yellow buses.
Any PVR savings from not having buses run dead to Hyde Road for breaks, would be negated by the fact you would need a regular staff shuttle bus, or you would be relying upon the normal Hyde Road service buses running on time and then drivers walking around the city to find their bus.
Finally, this way of working (given the depot location) means that if each driver is bringing a fresh bus from the depot, it can improve reliability as the driver going out can start on time irrespective of how late the inbound bus is running (of course depending on the number of other buses available).
That said, it was the case under Stagecoach that routes which go past the depot, have driver swaps at the depot, not sure if Metroline have continued this bit though.