I had a question like this the other day. The problem those
people who gave up their passports faced was people still classify
them as overseas immigrants, as opposed to Americans or Canadians,
etc.
For example, indians. Their accent, their skin colour, their
ethnicity would make one classify them as indians, even thought
they are already citizens.
ps: The going market rate for SG pp is US37,000. A well used, well
travelled one commands a higher price than a barely used one.