I went from having PMs to having no PMs to having PMs again. And I've officially confirmed my position that there's little to no reason for them.
I don't work in product design, but as an art director in an agency, PM's are so integral to my project workflows. I'm convinced that every designer who doesn't like PM's either never worked with a good one, or never worked at an agency (or both).
I personally have such a solid relationship with the PM's I work with at my agency. Boundaries are never crossed, and they bring a lot of structure to my work where I can focus solely on design while they focus solely on wrangling clients and ensuring things stay on track.
And that's not to say I don't know how to manage clients and only leave it to a PM. I'm always in the room and presenting the work and walking them through design decisions we make, but a good PM can ensure that behind the scenes, the client knows they are being heard.
Yeah, I don't think every environment warrants a PM. Some may excel with one, and some may hinder with one depending on the setup. Never worked at a tech company or startup so I wouldn't know how useful one would be there compared to an ad agency.
Especially since working at an agency requires me to be across varying projects and scopes in mind with multiple clients and working with their teams vs. working in-house that is more internal facing across the same group of people every day.