Not just safety
Having been based at LHR for 30 years, during which I operated to over 200 airports worldwide, I am very much in favour of the 'follow the greens' system. I might be thought to be, and probably am, biased by long familiarity. However, many of the points made on safety grounds are entirely valid. What may be less obvious is that the system also enhances the movement rate. During the period of bad weather in December LHR achieved movement rates with Low Vis Procedures in force of 28 to 30 per hour. There were howls of anguish at the delays and cancellations caused, how much worse they would have been without the greens. Even in good weather the movement rate is enhanced by their use because of reduced RT and fewer routing mistakes. Sure, the operator can get it wrong, that applies to any system. That being the case, the removal of a monitor is another poor move.
The size of LHR *is* relevant. As anyone who has operated there knows, LHR is in a permanent state of flux, particularly so with the advent of T5 and the massive changes to the taxiway system that involves. When that is complete the redevelopment of the Central Area will bring changes for many years to come. The confined space in which LHR operates requires that WIP areas are moved far more frequently than would be necessary were there more space for alternative routes. Even a pilot familiar with LHR can become confused by WIP. I have the T-shirt!