If there's a big group, like 10+ global mods, then I'd definitely see the logic behind allocating an admin to oversee global mods specifically. I dont think a global mod should be the "leader" of other global mods, since that's a little unfair and selective, and a breeding ground for envy and disagreements on principle.
All forums I've staff on haven't had the need for more than 10 global mods, though, so it's just been up to the admins to be the "team leaders" of all staff, which is the system I prefer anyway.
My (up and coming unreleased forum) does not have team leaders, and does not plan on implementing such in the future.
Dregran covered it covered quite well. If the team size is large, sure, a team leader is fine. If there are many tasks for the user group, a team leader is also great for helping to execute and organize these tasks.
I currently do not have or ever really plan to have team leaders. I find them rather useless the forum is huge and the administration cannot handle or communicate well enough with it's members.