"So why write G-d and not God?"<quoted text>
To make the distinction (given matthew 28:17-19 where jesus is revered as a god)
And some say it's a messianic custom, kind of acknowledging Ha-Shem and not jesus as god. Though i never got to understand the ratio behind that custom. Given that christians in general borrow verses willy-nilly to substantiate the gospel.
A form of respect. But that would in christian terms also be served by using the term 'father'.
I answered that question already, I think FR missed the post, and Hughe, in usual fashion is oblivious even if I did answer it.
There is no traditional (i.e. halachic) prohibition against writing God. The custom of writing it as a G-d instead of God is a modern custom. The traditional view is that you can write God because God is not the name of "God" - i.e. its an English, not a Hebrew term.
However, some Jews follow the modern custom as if it is a traditional prohibition anyway.
I believe the rationale in that case is the same "as if" God was the name of God (i.e they are extending the Hebrew concept to English), and that if you spell it out and it becomes discarded on paper, it is the same as desecration (which is the primary reason why you dont spell it out in Hebrew). Therefore, to be safe, those Jews dont take the risk of writing it. Speaking the English word God is not desecration (i,e, no act of destructing the name) so it doesnt matter if you speak it.
That said, it is usually not my "modern" custom to write God, I follow the tradition which does not apply to the English for the above reasons. However, there are Jews here that do follow the modern custom. Its an individual choice, I guess, whether you extend the traditional prohibition to include the English word and make it a modern custom as well.