OK, so "works" isn't the right work. This system is blatantly coercive. It takes advantage of the desire to do real, productive research and transforms that desire into raw labor. It's like a gold rush long after the rich veins have been depleted: you're still left with a bunch of laborers who can swing a shovel but won't strike it big anytime soon. Why not put them all to work more-or-less indefinitely? Otherwise, whole branches of science become ghost towns (though that's happening as well, and for similar reasons).
Part of the issue revolves around what postdoctoral researchers really are, at least in an official context. The NPR piece puts it like this:
The entire system is built around the false idea that all these scientists-in-training are headed to university professorships.It's true that postdoctoral positions are intended as training. The NIH says they are "...engaged in a temporary period of mentored research and/or scholarly training..." as a means to a career goal. The postdocs I've talked to have expressed how this leaves them stuck between the role of a student and that of a mature, capable researcher. At universities, this means they don't get any of the benefits of studenthood (i.e., acknowledgement that specific training goals need to be met) but they certainly aren't university staff. It's often unclear who they really work for or whether they're even employees.*
In the end, we're left with more than 40 thousand people forced to sacrifice their intellectual and economic independence for years on end. This isn't just about the value of a doctorate. It's about a massive societal disconnect: science is something that everyone wants to do but no one wants to pay for.
Somehow, I'm remaining optimistic.
*That article is from more than a decade ago but I believe much of it still applies.