some of the votes give the winner only, approval bonuses, etc - so, its worth staying in - as well, you can use your voting power to shoot down anything that will help your primary enemy, over the next 50 turns.  russia and china have stayed in the UN, for similar reasons.  the only reason I can see leaving is if vote after vote, you get nothing but have to pay taxes on it....

thats what North Korea said!!!

And all the members of the Security Council. And all the non-security council members. In fact, it's kinda of the whole point of the UN, really...

And the best reason to stay in is the resolution that keeps other races from colonizing planets inside your zone of influence.

I've seen the Drengin leave.

I was under the impression that if you weren't in the UP then you could not trade with players/AI who were. 

I've seen the 'bad guys' like yor, drengin, and krynn leave in many games.  As I play a good guy, they never seem to like me anyways and war is inevitable, so I don't  setup trade routes with them, ever.  i am pretty sure that you can trade techs with them at any time unless you are at war with them even if not in the UP, like the norm.