Ok, instead of saying what does a Global Chat harm, try to say why Global Chat should be implemented.

For all that I know from this topic:

YAY - Helps Trades and Party-finding, also interact with other people on the world.

NAY - Disturbing, annoying. That would let everybody toggle Global Chat off and never use it.

For the things so far, My mind says: We don't need it. The moment we'll need it, they'll implement it, and we know when we need it.