I'm a product manager for a 16 person team. I define the requirements for our products (http://www.staffsquared.com, http://www.fundipedia.com), check to ensure they're produced to the highest quality holding our developers and testers to account, and ensure that our product dev is aligned with our sales and marketing efforts.

I feel your experience of the PMs is specific to that of a billion dollar company with more resources and A star employees than most companies could ever dream of. In smaller companies that enjoy revenue in the millions per annum, I can assure you that a good PM is a fundamental contributor to success.

I guess what I'm trying to say is, you can't write off all PMs. It's all about context, and perhaps in your world PMs are garbage, but in smaller teams they're absolutely essential.