@TSR2, those holding areas are normally empty stands. But if all the stands are full, where will you put the aircraft? There isn't enough space to put them at the runways, you could put one at U1 for 23L maybe, but if you're on 05s or 23R single there is no space (the JS1 holding bay is gone).
Manchester is not a place with a load of spare taxiways, you get room for a few aircraft to hold, but you quickly run out of space.
Also, since two taxiways (Q and P) double as stands, when there is a lack fo stands these are already closed, meaning you get even fewer places to hold aircraft waiting for stands.