The benefit of this kind of consanguinity is that when coupled with selection, deleterious alleles can be weeded out of a population.

When I was in the Army in Kentucky I met real hillbillies. They in fact did talk about shooting at "revenuers" and running moonshine. But you didn't have to listen to them to identifying them you just looked at their teeth. If the guy had a mouthfull of rotten stumps you knew he was descended from a long line of first cousin marriages.