Good marketing isn't about your product and its features. Good marketing is focused on solving a problem and then helping buyers buy. It's not a list of tactics, but an approach to putting the customer first in your thinking. Not "We should be blogging," but "Our customers are searching blogs for information about our specialty."
So product marketing managers should not be responding to individual Requests for Proposals (RFPs) or invitations to tender; they should create a sample document with great answers to the most common questions. Product marketing managers shouldn't be doing demos; they should create great demos for the sales team to use. They shouldn't create presentations for a single client; they should create presentations for use at every client.
Great product managers find patterns in customer requests and build features that address urgent problems for the buyer. Great product marketing managers find patterns in the buying process and build tools to answer buyer questions.