Ah, ok, I misunderstood you and it obviously touched a sore spot on me.
I will say I disagree that unless they're specifically barred they're OK, I think the bar you have to pass is "reasonable expectation". I think that if nothing is said when the victory conditions are specified it's implied that each player is playing for a single nation win, playing only one nation, etc. etc.
I think the possibilities of real alliances are very interesting, and I hope to see more team games like the Allegiance MP game. I've had a lot of fun working with my prearranged ally coming up with two builds that complement each other and an overall strategy we're working toward (splitting up research goals, conquest priorities, etc.). I don't really want to go into too much detail about our secret plans in an ongoing game, but it certainly is a different game played that way!
I also think some of the more interesting diplomatic dynamics are between long term allies that know they'll eventually have to fight each other...if they last that long!