I used to be a product marketer. My ideal working relationship is to view product marketing as my key partner in product development, launch, and iteration. We are both stewards of the product success.
I try to involve product marketing into the very beginning stages of product development - they help me amplify the voice of the customer, help me find reference customers by launch, fine tune messaging, speak to industry analysts educating them on the new product, enable sales and customers on the product benefits and messaging. PMMs also conduct competitor messaging/positioning analysis, deal win/loss analysis, and in my experience, how the product is represented in all facets (website, content marketing, customer facing slides, analyst research).