I agree that a PM who simply play the role of a telephone between design, engineering and marketing is indeed useless...though i disagree that PM in general add no value building product, even in a team of A players.

You work at google, and google is a place full of A players. It also happens to have hundreds if not thousands of product managers. Surely they are not there to just look pretty.

I can't speak for you, but from my own experience, I get into the weeds with engineers on technical tradeoff and debate over user experience with my designer on a daily basis. I run user testing, I scowl over metrics to understand what's working vs what's not. If that's not contributing to the product in a meaningful way, I don't know what is. I hope at some point, you get to work with a PM that is able to do that with you.