I use two spacing simply because that is what I was taught in school. After having it drummed into my head for the better part of 10 years, I'm never going to change for anyone.
"Gay marriage? After having it drummed into my head for the better part of ten years that it's a bad thing, I'm never going to change for anyone."
If you don't pause to re-evaluate your beliefs once in a while, you're doing it wrong.
I can't remember what my college course required because I only had to go five or six times to prove my proficiency. (Yes, my university's computing program required a typing course because they had students that couldn't touch type.)