To put it a bit more simply than Robert
Anyone who knows anything about breeding animals will tell you that isolated genetic populations are always inferior to widely crossbred populations. Think dogs, or royal lineages.
Hybrids (and I use this term very loosely here when talking about human genotypes, which are not that different) are the most robust in genetic terms.
Generally speaking, for a healthy human race, the more cross-breeding around the world, the better.
Makes for hotter-looking women, too.