I sing it because, regardless of the words, it's the anthem of my country - I disagree with the words, but I don't really feel anyone pays attention to them in the first place. I'm perfectly happy for him to not to sing them, though.
I do draw the line at England using it as a national anthem, it's the national anthem of Great Britain - the other component countries have their own anthems, I don't see why England thinks that it's special.
O.