Think about it this way, that it takes only ONE person to mess it up.
There are many companies that demand that you not work anywhere else, while you are employed by them because your motivation or loyalties will be divided.
America pays you more, treats you better, helps your children go to school... Now America goes to war and calls upon you to fight Pakistan. Pakistan calls upon you to fight America.
Since both countries have a draft system, you must comply.
Who do you choose?
Hypothetical scenario, but the answer is easy: I would choose Pakistan. But then, I am not an American citizen, nor do I wish to become one, and neither did I swear an oath to defend my adopted country in the event of war. (NOTE: I am not advocating fighting America, just making a point about loyalties!)
A time of crisis certainly would test loyalties considerably, but that is true of life in general. Better in such circumstances to ask what does Islam require you to do - after all, if you die fighting for the wrong side, you have to face your Creator and answer for your deeds. Given that not everyone would necessarily take this train of thought to its logical conclusion, chances are that civilians forced to choose might simply resort to draft dodging and escape to a third country; loyalty to the self can easily trump loyalty to the nation-state.
To avoid this risk one must belong to only one country. You can still keep helping Pakistan out as an only American citizen, thats up to you, no one would stop you but at least you can't enjoy equal parity with a guy who will ONLY fight for Pakistan and does not have their loyalties divided - at least on paper.
In peacetime, and in today's globalised world, belonging to just one country is tremendously restrictive. Free movement of people, goods and monies makes for a healthy and better world; indeed this was the case throughout human history until the Europeans invented the concept of the nation-state at the Treaty of Westphalia, and then exported/imposed it upon the rest of the world during the era of colonisation.
The situation is quite different in wartime, I agree. Dual nationals can easily become patsies for infiltration and other wrongdoings. That, however, does not negate the tremendous good that dual nationality can bring in peacetime. As such, the concept itself is not a flawed one, although its implementation in certain situations can be. Hence, dual nationals should not be eligible for public office or military service (Pakistan already requires the latter) - that much I can agree with.
