Why is assimilation bad?
Not everyone would say that assimilation is bad. If it is bad, it is because it is important to retain one's culture and values. You could argue that this is important so that there will continue to be cultural diversity in the world. You can say that there are things of value in every culture and we would not want any cultures to die out lest those things be lost. If we allow cultures to die, we become too homogeneous.
You can also say assimilation is bad because it implies that the main culture is superior to the ones being assimilated. This can tell the people being assimilated that they are not as good. This can lead to them feeling negative things about themselves and their ancestors.