I agree that a product marketing-minded person is very helpful in the early days of a startup. PMMs tend to be good generalists because we own messaging and go to market planning. We also need to deeply understand the market, product, and customer base.
In the early days as the first marketer, you need to not only make the plan but deliver the plan completely on your own. From writing content to creating emails and ads, it’s all on you. Having a strategic mindset for customer needs, market context, and messaging can definitely help here. That said, don’t expect to only do traditional PMM tasks as the first marketer. You’ll have to dabble in demand gen and brand as well.
If you're on the hiring side, make sure that the PMM is willing to manage all of these end channels as well as the message. If you hire a PMM first, the next best hire is usually someone more growth-oriented.
I was the first marketer at Segment. I actually came from a public relations background and morphed into PMM due to my proximity to customers and our product team. I found I liked PMM best. PMM and PR minded folks are likely strong on the content and brand side but may have less experience in demand gen. In the early days, demand gen is also very important unless you are blessed with viral word of mouth growth.