People who spam global with that are usually looking for something specific and there might be someone willing to sell exactly that without including the rest on the char. And that might not be available on trading at that moment.
Global can still be used to buy/sell items cheaper or faster than on auction house, without the 5% cut and without waiting 1 hour for gold.
I agree with this one since repeated guild recruitment spam is just annoying and the messages are usually long and take too much space in the chat.

