Yeah, my mom only got US citizenship when I was in high school, & that was only b/c some law changed that would fuck up her social security benefits. She was quite happy to regain her Finnish citizenship when they changed their laws to allow her to do so (& allow people like me to gain it). An old boss of mine (British, actually) has lived in the US for almost 20 years but has no desire to become a citizen. And of course there are the undocumented, as well, for whom there's v. little choice in the matter. (& DREAM Act kids, for whom there might be a choice, someday...)

I can't imagine ever wanting to acquire British citizenship, no matter how long I live here, but then it's easy not to b/c I have EU citizenship & thus a certain amount of privileges anyway (can't go on the dole, though).