Having one religion is like everybody having the same favorite sports team, the fun is in the controversy.
Yes, but while it is fun for some, there are these fanatics who resort to violence to those who disagree with them. Personally, I don't think religion is needed at all in today's society. However, if people have to follow a religion, I think it having one would be preferable to having many, since that would then the fanatics wouldn't have a reason to try to convince others to follow their belief by doing the exact opposite of what it teaches. Hopefully.
Though, since there are already many religions in the world, I don't think people should convert from one to another just for the purpose of uniting everyone under one religion. That'd be silly.
But, you know, people interpret religions in the way that suits them best, so most people would have a different interpretation of that one religion, which in turn would most likely cause the same kind of violence that was the only argument for having one religion in the first place.
So, my opinion? If people would just believe in what could be proven, they wouldn't have to fight over things that have no answer.