A Singaporean says in ST Forum that we should learn to pronounce "Singapore" correctly. And I am with him.
However he goes on to say how it should be pronounced as "singer", which threw me.
Where I live one often hears people pronouncing 'singer' as "sing-ger" and 'singing' as "sing-ging" instead of what I would say as "sing-nging". (And 'drawing' is pronounced "draw-ring".)
I am almost certain that I am pronouncing it wrong as I often do, but my 'singing' sounds a bit like on this link: http://www.howjsay.com/index.php?word=singing&submit=Submit , and I believe this is the way the writer meant.
Interestingly on the same site, you will also find "Singapore":
which gives an alternative "American" pronunciation. Here it could be "Sing-nga-pore" or "Sing-ga-pore".
I am 'old school', and with a legacy of Bahasa Kebangsaan would prefer to say "Sing-nga-pore", which means that it is not pronounced as written. I think it sounds more elegant, less vulgar. But that's only my opinion.
I worked very briefly in Jakarta and noticed that a particular person in the office spoke Bahasa Indonesia in a very lilting manner. My colleagues said it's because she's from a certain part of Indonesia (eastern Java? I don't remember exactly) where they spoke more gently there.
My conclusion is the letter writer's quarrel is not "sing-guh" or "sing-gah", but the writer is asking for Sing-nga-poreans to use the more gentrified(?) 'ng' sound.