Not sure what the new data here is, but I think this is an important point worth promoting. Perhaps it's the geographic association they show. In that case there could just be a confounding correlation: areas with more dynamic economies will show more mixing, and less dynamic ones will separate with the cream rising to the top (and curdling ;-P).

My guess (and experience), is the these cross-class friendships are most important for networking, and the biggest single benefit of that is that it enables people to interview for jobs that they would have otherwise been screened out of.

Since, IMHO, this is a mostly zero-sum effect (though of course there is some value in trusted references), I think it would be good to try to reduce the impact of this in our society.

So… friends in high places?