I'm waiting for the disruptive startup that revolutionizes corporate heirarchy by getting rid of PMs since they're so useless.
Since successful businesses continue to employ them, I'm going to err on the side of they do serve a function I don't always understand rather than they must be stupid.
Let me introduce you to the glory of Valve and Lord Gaben.
On a less snarky note: Depending on organization type PMs might do a ton of paperwork on behalf of the teams and act as some sort of central point for business-product communication, however that has exactly nothing with management in any typical definition.
Since successful businesses continue to employ them, I'm going to err on the side of they do serve a function I don't always understand rather than they must be stupid.