Well... If an immigrant, about the only things one gains from citizenship in the US are the right to vote, and freedom from deportation. Immigrants tend toward being Dems (although that is certainly not true of all!), so persuading them to remain green card holders is not all bad. Plus, some want to retire to their birth countries after their working years, and, that is not all bad either. Heck, finances alone have me wondering if I, a born in the US citizen, should head overseas once I’m no longer able to work effectively. Otherwise, I see no scenario where I do not have to work until I drop dead.
I have dealt with people who were part of Reagan's amnesty and for 25 years hadn't applied for citizenship even though they reaped the benefits. Free healthcare, welfare, etc.
Plus, originally they were encouraged to become citizens, but because they were too lazy and the fact they had to learn a modicum of U.S. history, which applied to the laziness, they neglected to become citizens.
For those people, I say, "T.S."