NS is conscription: forced military service. Anything that's forced upon a person is never a good thing, because free will has been overridden.
Now, you can say that you can 'learn to enjoy' NS, or 'make the most' of your time in NS... but the same can be said of prison time.
NS provides great business for the GLCs and contractors, and NS slaves are an abundant source of cheap labour who get activated for things ranging from ushering duties at LKY's funeral to doing chores for the annual NDP.
That is the true reason why NS won't be scrapped, so stop using the foreign invader bogeyman to scare us.