If the UN were the organization that we wish that it were, this would be a fine idea.
Unfortunately, the politics of the UN would likely create a complete mess of the whole thing, resulting in the creation of a parallel system as folks tried to route around the problems caused. They don't have the best track record with managing things ( UNICEF is probably their best managed organization, and since it is ostensibly a children aid charity, it is difficult to politicize what it does (ie, it would not look good to deny aid to a country's children as punishment )).
Just look at the mess many ISPs have created by abusing their DNS services, and the rise of OpenDNS as an alternative to avoid them.
If the system were broken, then by all means change it, but since the choice here seems to be to take your analogy, one of taking my money management away from the accountant who hasn't screwed me yet, and giving it to my idiot neighbor who constantly writes bad checks, I'll pass for now.