Multiculturalism in societies fosters understanding among people of different ethnic backgrounds. Acceptance of different cultures prevents fearmongering and disenfranchising of certain groups. Assimilation is too forceful and too abrupt, even if it means becoming one with another culture in a strange place. Multiculturalism is the only way to attain world peace. Unfortunately, that will take awhile with so much ethnic strife in the world today due to economic pressures.
Assimilation provides for multiple cultures to develop into one combined culture. This would provide for better protection of individual rights. This is because if everyone was similar then they would all feel equal. Multiculturalism has the unintended consequence of separating and dividing people. This leads to potential racism and denigration of individual rights.