I'm a dual Canadian/U.S. citizen, although I was born so in both countries (born to American parents on Canadian soil). I've never once had to worry about this whole renunciation thing.

Additionally, my brother became a Canadian citizen (born U.S.), and again had no problems; Canada didn't care about his U.S. citizenship, and the U.S. doesn't care about his Canadian citizenship. He could officially renounce his U.S. citizenship by walking into a consulate and doing so, but there's no up-side to it.

There may be some upside to renounce a citizenship to do with avoiding extradition. But unless you plan to get involved in criminal investigations, it's probably not worth it.

If America would get a draft again, could a dual citizen be drafted?

Yes; as far as the U.S. is concerned, you're a citizen, with the same rights and responsibilities as any other. If there's a draft, you'll be called upon.

If that were to happen, there would definitely be an upside to renunciation. ;-)

Just watch out for when both sides call a draft.

And worse, go to war with each other. It happened back in 1812 you know...

Then you are a double-agent -- for both sides.

This is a problem for Turkish man living in Germany with dual citizenship.

Some countries don't allow dual citizenship. My German friends had to renounce their German citizenship when they became Americans (again for the green card). It made it very tricky tax-wise when their parents died and they suddenly became foreigners (to Germany) owning a lot of German assets.

as far as I know, it's possible to apply for keeping your German citizenship when gaining the one of another country. you have to justify it with family relations or something like that. at least it's possible, but I don't know about the trouble you have with that ...

Indeed. You also keep your German or Austrian citizenship if you did not apply for the other citizenship (e.g. if someone was born from German parents in Australia).

In general different countries have (very) different sets of rules, see e.g. http://en.wikipedia.org/wiki/Multiple_citizenship

If you're born a dual Citizen then it's not a problem. If on the other hand a non-US citizen applies to become a US-citizen then they will be asked to "renounce all other allegiances". However depending on what your other "allegiances" are the US will not bother too much about following up on it.

Also if you volunteer (as opposed to being conscripted) for military duty in your non-US nations then the US can kick up a fuss.

But basically if you're a dual citizen with a nation friendly to the US then you really have nothing to worry about.