I am not going to answer the weapons debate about halberds but the game mechanics, specifically balance.
One of the previous users said its to give GK a bonus against the faster squishy units that can kill them first in combat. The army already has tons of super high strength shooting, meaning that those weaker faster units are doomed before they get there anyways. Its really hard to actually get into a combat safely with the grey knights in the first place! Why do they need to make an army that out does all others in each phase of the game? It should be that if you are charged in CC then you are doing something wrong tactically and deserve to get butchered.