Considering that global staff tend to have their PMs blocked (for various reasons) and there's not really a reason for someone to be PMing random global voices, I'm not sure I understand the use here. Vast majority of the time, a person should be opening a help ticket (or worst case, visiting the Help Room) if they need global assistance anyway, no?
E: Another thing is that when you try and PM gstaff who have their PMs blocked to any rank higher than yours, the system will tell you to open a help ticket anyway.