It can cause you to see other ways of seeing things, and you'll question things that you've accepted as gospel your whole life. Many people find this disconcerting and threatening.
When there is a diversity of cultures in a region, you can have ethnic and religious conflict as a result of these different cultures. Diversity is only a strength if it can be utilized to the nation's benefit. The Romans absorbed diverse peoples then used those individuals to help conquer neighboring tribes. A monolithic culture by default has unity.