I once heard this rationale from a warrant officer, it was quite
some time ago so I can't really recall much about who told me and
what the broader context was.
However, the rationale was regarding why the SAF usually didn't
send Malaysian & Indonesian Permanent Residents or those of
immediate Malaysian/Indonesian descent to sensitive
vocations.
The rationale was that although these groups of people might be
eager to serve and might bear a loyalty to Singapore, they are
still considered an "at risk" group since they probably still bear
strong family ties to relatives still in their former
country.
If worse came to worse, and war became a real possibility, would a
person in these two groups, obey orders and fire a weapon at a
particular area if at the same time, he's worrying about the safety
of relatives nearby? Would such a person risk hurting his own
family to protect his country?
