I am reminded of Orwell's 1984 with this question and the responses to it. All countries merging into just a few would not be a good thing. It would severly limit personal rights, as the governments that result would likely be too powerful. On the other hand, I think that as a global community we can no longer plead ignorance to what happens to our fellow humans. I think that the situation in Egypt is proof that it may no longer be possible.
I think you could also make the argument that the UN hasn't done work on par with the funding and effort it has received and has often actually made certain situations worse.
I think you have to remember that it is dangerous when people who aren't familiar with a situation decide they are going to make decisions for communities or nations.
I agree with Pohnpei. In addition a loss of sovereignty countries might also lose their autonomy. Their ability to make decisions will then be limited, and then rather than being unifying, the process becomes divisive. In addition, countries might begin losing those very elements that makes them unique and appealing.
If you are using the term "internationalism" to mean that countries will cooperate more both politically and economically, the major negative impact is the loss (partial or perhaps even complete) of a country's sovereignty.
A major example of internationalism today is the EU. They are trying to create a system in which their members cooperate very closely. However, the EU is pressuring its members (like Greece and Portugal and Ireland and Spain) that are in economic trouble, trying to force them to reduce government spending in order to get out of trouble.
In a case like this, a country's ability to rule itself is somewhat in jeopardy. The more internationalism there is, the more countries can be pressured to do things, even in purely domestic matters, that they do not want to do.
@kapokid, but wouldn't the alternative be to let matters slide and to ignore it? i mean, places such as Darfur and the Ivory coast comes to mind; where the UNHCR has protected refugees and in the Ivory coast an opposition politicion.
but, other organisations like the UN, have done good work haven't they? With UNHCR and the security council, haven't they progressed the world furthur in terms of international relations? and human rights? In order to achieve a greater good, we must put down our national interests. Isn't that one of the basis for internationalism?