Is it important? On what scale are you talking about? How far back historically? Do you think it's a great loss that the Spartan culture and Athenian culture and Macedonian culture have all become one Greek culture? Or that there are no more Visigoths, Ostrogoths, and plain Goths? This is what history is.
I think your argument is more sentimental and emotional, and that's why you're NOT going to find a lot of factual reasons--there really aren't any.