I don't know why but Yahoo Publisher ad targeting sucks big time. I had the ads on one of my sites but had to remove them because they were not related at all to the topic of my site.
I had the ads for a full week and only had one click.
In comparison, I do get at least 5 clicks on that same site with Google adsense.
Even if people say that Yahoo Publisher pays out more per click, I don't think it is worth putting them on any site until they fix their ad targeting.