This is really a metric we want to keep significant? "We'd just feel more comfortable giving our good jobs to people from already well-off families; if you're not from one of those, tough luck..."?
If you're running a welfare program, that's an unfair criterion. If you're running a business and need competent employees, then it's not a bad screening measure.
I recall the blue collar employers questioning me about my choice to interview as they saw me a s "flight risk" that I'd find it beneath me or something --having been going thru Uni, or that I might not fit in with the rest of the shop workers.
Basically, interviewers want some assurance of some type you will fit in, whether you'll be able to blend in with the rest of the employees -in the blue collar places, that I might fit in with a less polished crowd and whether one might find it fulfilling.