When I meet someone with two PhDs in Europe I started to evaluate if they're contributing massive brain-blasts to civilization or if they just wanted to delay the next chapter of adulthood and use free education as a camouflage.
A Ph.d is mostly cheap labour for the relevant research institution. I wouldn't count that as a cost to society unless the work done is not useful, in which case it is not the Ph.d graduate but the research institution that funded the project that is to blame. Get a Ph.d position is quite competitive and the ones which are less "useful" from an immediate economic perspective are paid much worse.