Trick with Yahoo is to keep an eye on exactly where the traffic is coming from.
When they turned off their "content network" what actually happened was a mass reclassification of what fell into content, and what fell into search.
With the right tracking you can understand exactly where the bad traffic is coming from, and block it by IP. That way, you can slowly improve the results over time. As networks combine and develop, this level of visibility becomes more and more important.
Obviously, you will need to decide if this is a good investment of your time, or if you're better off just focussing on Google. However, with more and more people deciding to do just that - there becomes an increasing opportunity on the smaller engines (or, engine - as is soon to become the case).