Multiculturalism's positive factors:
-Allows people to practice their religions freely.
-Makes a country more unique and diverse.
-Does not involve forcing anyone into another group/religion/culture.
-Makes a countries quality of life better.
....There are many other positive aspects to being a multicultural society, but these are just a few that have been listed.
The United States is a country of immigrants, and I feel it's important for immigrants to keep some aspects of their culture. However, it's also important to remember that cultural differences may block those outside that culture from being involved, so we need to work at also assimilating in some respects, in order to speak the same language, be better able to seek and find jobs, etc. It really should be a mixture of assimilating with society, while also feeling free to practice aspects of one's own cultural identity, as well.
You have both happening. Children who's family has been in a country for generations will begin to assimilate. They will take on the language and culture of their new country and get further away from the culture of their family. And as more people begin to intermarry with other cultures, there is room for both.