On the nicer carriers (which don't upgrade passengers for free and charge a lot), the curtain is used to keep the cabins seperate and private. I think that is the intent.

I don't understand why US carriers don't like to use the curtain. Swiss, Singapore, Lufthansa - all use the curtain, to and from the US.

On Singapore Airlines there is a sign up above on the railings- "First Class Only". Even when the cabin is empty, the curtain is pullled (its part of their operating procedure) and generally you are not allowed to go to the First Class Section, unless you hold a First Class Boarding Pass.

If you have a Business Class ticket (as I did) you stick in Business, though you can walk around to the coach/economy section as I did.

On Qantas, they don't like you moving from one cabin to the other. For example if you are in First, you stay in First.

I love the curtain. It keeps things "civil".