If a country is capable of insuring the safety of their artefact's from past civilizations, then those artefact's are best in their original geographical areas with the local cultural contexts. While it would be nice to ensure the largest number of people would be able to view those artefact's, unfortunately, that tends to land items in areas where on the richest people of the world could view them.
Cultural treasures and artifacts should go back to country of origin due to the fact that they were the countries property in the first place. Take an artifact from Egypt for instance. More than likely it was either buried with a persons entombed body at the time of burial or it was found during a dig. If you take that away from a country, no matter what its value, it is stealing. The people of the country would treasure it far more than any other country. It shows their culture, their heritage, their way of life both past and present, not to mention what the significance of the artifact/treasure would be. If you took the Constitution of the United States out of the United States and displayed it in China, would it have the same effect on people in China as it does here? No. Same thing when it comes to the artifacts/treasures of other lands, no matter what their real significance is.