The problem with Dominions, Diplomacy or playing sports for fun is that on the one hand extremely competitive people can be unplaasant to interact with, while on the other deliberately uncompetitive behaviour tends to subvert the whole purpose of the game. Both tend to increase the frustration and reduce the fun for the rest of us.

Sand-boxers may disagree, but it's not like we're talking about simDomII here; the central assumption of the game is one of conflict.

I don't think alliance "victories" are necessarily wrong. Often alliance are result of in-game events and built up around developing relationships. Maintaining an alliance amongst competitive players takes effort and skill, and there's nothing wrong in rewarding that by allowing joint "victories". I can see however that if alliances seem more-or-less pre-ordained by pre-game relationships, or if a large number of players in a game just weren't interested in competing with each other, that I too would get frustrated.

I used to see a lot of joint victories in Diplomacy when I played, and never objected to them because the games were always competitive, even though technically those results went against the intention of the game design. On the other hand, sand-boxers wouldn't be playing Diplomacy in the first place.