Internal marketing is still marketing, especially if it potentially increases the value of the site via increased user engagement. Marketing is a very broad term and covers everything from promotion and sales (both internal and external) through to things like market research, branding, and so on. It's a broad enough term that definitions tend to vary
One principle of marketing is to predict consumer behaviour, not just react to it. That said I fully understand not wanting to create forums that may just become tumbleweed zones.
The DAM generated a whole lot of interest and some very long threads, while the Spyder Hammer7 was similar.
I would suggest that the interest for magfed *play* is definitely there (and growing), so it follows that a forum would work. Part of the issue however is that pbn is often seen as a speedball centric forum so the bulk of relevant users may not be here yet. Once word gets out things could pick up but it might be more through new users than existing members.