Hi BlueEagle,
I'm not sure what you say is correct. My understanding is that if you can prove that you will be under full employ (i.e. Letter of employment) to work overseas by a firm, your NS liability will usually be put on hold. Untill such time you either come back, or you exceed the NS age limit.
I hazard to guess that the reason why most SIA local pilots choose to stay on in Singapore is because of family commitments and also because of property liability. As you know houses in SG are rediculously expensive and most Singaporeans use part of their compulsory superannuation (CPF) fund to offset their housing loan. If you work overseas = no superannuation = pay out of your own pocket. The loans can take up to 30 years to fulfil.
Also, unlike expat pilots who are used to roaming the world in seach of better prospects, the SG local pilot isn't like that.. culture I guess. It's out of their nature to pick up the whole family, pack up and go to somewhere else where the pay might be better, but the move will disrupt their child's studies etc etc.
So they're stuck between a rock and a hard place.
But hey it's just my guess.
Ghos