I'm not so sure. You could argue that it might increase it. Most businesses here don't advertise via the ad slots because it's more effective to pay users. It might even be cheaper in some instances. Also, theymos doesn't seem to care that much about forum income so traffic probably isn't that much of a concern either. At least if sig campaigns were banned then the quality of discussion would go up exponentially. Though as I've always said, quality discussion and sig campaigns can coexist with better management on their behalf. Imagine if every campaign was run like Darkstar's. We would then have no problem with spam and they would actually help improve the quality of content.
I have long argued that companies should be called out when their advertising is harming the forum in the form of low quality posts.
As a forum user, I very much appreciate how Darkstar runs his campaign because of how little spam his campaign contributes (although there are some in his campaign that can both post without knowing what they are talking about and get merit in these posts). However running a campaign the way Darkstar runs his campaigns is going to be more expensive than running a campaign the way stake ran their campaign both to participants and to the manager. If there were tangible reputation consequences to having a campaign run poorly, more companies would have their campaigns run like Darkstars.
I have also noticed a trend that companies are starting to pay people to blog and write articles about their company on various other platforms (including other social media sites). I would suspect that more companies will move toward this type of advertising because the content stays up (usually) after payment has been made, and the reviews help with google search results.