Or, as the D&D splatbooks put it:

Savage Species:

Evil is not stupid. Evil creatures and characters can work together just as well as good characters can. It should be no more difficult to maintain party cohesion with a group that includes LE, NE and CE characters, than a typical adventurer mix of LG, NG and CG. Certainly evil characters attempt to manipulate events to their personal advantage- a phenomenon not limited to evil parties- but not to the extent of sabotaging their own chances of survival.

BoVD:

Two evil PCs do not have to come to blows just because they are both evil. Evil characters with similar goals or common foes can certainly work together. And there's no reason to believe that evil characters can't respect something like friendship. Intelligent evil characters realize, just as nonevil characters do, that they can accomplish more by working together rather than working at cross-purposes.

Champions of Ruin:

No-one is going to question whether the priest of Cyric is really a good guy at heart; he isn't- if he were, Cyric wouldn't grant him any spells. But just because he is evil doesn't mean he is going to slaughter his companions and steal their treasure at the first opportunity. If evil were really that self-destructive, good wouldn't have nearly as hard a time combating it.