If you're a startup with lots of possible advertising topics, and monetizing with advertising, you're probably a UGC/social media site. Advertisers will see you as not much more than a glorified general discussion forum, and pay accordingly.

Forum traffic, unless it's very targeted to a specific niche, does NOT convert well. You won't get a good CPM no matter what. The exception to this rule is for social networks, who know enough about their users to allow demographic targeting. Even then, the CPM isn't great(I pay less than 20 cents CPM on Facebook for example). Social networks make up for this on volume, because social apps tend to generate a lot of pageviews.

That said, you will still do better with ad sales people even if you're selling general, untargeted inventory, especially if you're selling to brand advertisers.

Every couple weeks I get pestered by ad sales people from some second-tier social network who are willing to do almost anything to get an IO and commitment for a large number of impressions. Sometimes this works, even for low-quality traffic. It's possible to sell almost any kind of traffic as long as the numbers are right, and the benefit of having ad sales people, at least good ones, is that they can sell large blocks of inventory to big advertisers at once and hopefully maintain a relationship if the traffic backs out.

As for Demand Media, you're wrong about how banner buys work. If I go to cracked.com, which has lots of diverse content(and low-quality traffic), I don't see ads that are targeted to the categories or specific niches. I see banner ads from big CPG advertisers doing branding sitewide.