I think it really depends how loyal your members are. My current forum is really active, but the top 10% of my posters make 80% of the posts and I have a very high return rate (which is good and bad). My traffic is really targeted to the person who will register and start contributing. If I were to implement Kontera style advertising, I'd definitely offer a selectable style theme for members that doesn't have Kontera activated.
It really depends on the types of community you have.
I think on a forum that gets a lot of transient people that search queried to your forum to find out how to fix a leak in their 96 Ford Taurus real quick, Kontera is especially successful.