It would be great that dual cab and hst type locos have engine room entrance. Passing from one cab to other through engine room without jumping down the cab and passing from passenger car to locomotive directly in hst would be great. There may also have some active controls which are carried out in engine room. Dual cab electric locos start up procedures, cab selection, pantograph selections .. etc may also be included with engine room access.
Sadly that logic doesn't really apply to GWE as the only dual cab locomotive is the 66, which has a radiator section that you can't walk through, so going into the engine room can't get you to the other cab. In fact, the number 2 cab doesn't even have an engine room door.
TrainSim-Steve said previously, on train-simulator.com, that it would be rare for HST drivers to enter the engine room and they wouldn't pass through it to change cabs. As such they left the engine room out. It also saves on a few frames which is nice.
I would by far prefer a new train for GWE, say the Class 800 or Class 387, than an engine room that I entered about once