Ethnocentrism can be good for a given culture because it can help to cause that culture to stick to its values and beliefs. If a culture is not in some way ethnocentric, it can fall victim to relativism. For example, if Western culture were not ethnocentric to some degree, we might lose our belief in individual rights. We might become more willing to allow authoritarian governments to take away our rights as is done in places like China. We might also be more likely to allow newcomers to our society to engage in practices we find barbaric, such as female genital mutilation.
If a society does not think of itself as in some way superior, it will not be able to hold on to its core values. In this way, ethnocentrism can actually be good for a culture/society.