This is a tough subject for me. I was an immigrant at one time and my family are one now in Canada. I was proud to become an American citizen but that was not to say I wanted to turn my back on the country that provided me with the morals in which I stand on for.
Canada and the US are immigrant nations, but for some reason in todays world we wish turn our back on the people who made up these nations. With that statement, don't get me wrong. We should do EVERYTHING in our power to vet anyone who wishes to immigrate. We should only except individuals who only have the best intentions towards our countries.
I understand the intentions of the "one flag" thing but that's overkill. When my family relocated back to Canada, my girls and wife were immigrants in a new nations and there we those few who looked down on them for being American. I bought a large American fridge magnet to remind my daughters that they could be incredibly proud of the country they were born. My daughters have grown now but that flag remains.
My family and myself are proud American-Canadians.