It depends on the difference, because genetic differences aren't always beneficial. The Hapsburg Dynasty's Charles II famously had an inherited birth defect due to the heavy inbreeding[1] among the ruling class, for example. It was of no value to anyone.
The true reason for valuing lineages is more about social rank/class protectionism.
The value of that intermarriage was ability to maintain an empire of significant power and influence, as well balance relationships with nearby royals, who were all cousins.
"no value" here is probably in tens, maybe hundreds of trillions of dollars in the present time PPP basis.
