DaasYochid – no, I was not suggesting that testing doesn’t help. As for the second, if the only goal is avoiding genetic mishaps, a more diverse population can only improve things.
But back to the original topic, testing, screening, and family questionnaires to make decisions about marriage. You can expend a lot of effort looking for a partner with no history of problems, but there is never a guarantee. Most important is who you will share your life with.