Jewish answerers have given the reason for not using the name in its entirety.
As far as why the vowel, I've heard it explained that it is because ancient Hebrew *has* no vowels.
I'm not any kind of monotheist, but when I am writing of Judaism, I will write "G-d", for Christianity "God", Islam "Allah" and for myself I speak of "my gods".
I use them simply as a sign of respect for that religion's conception of their deity. It's *their* deity, not mine, and they have a right to declare what is the proper way of showing respect.
hard polytheistic henotheist