Your story is wrong...LOLyahoo and msn is for optional gravy money. ppc pros make the bulk of their income from Google. End of story.
Yahoo does not have as much traffic, but you can get lower cost clicks, (if you're after anything below $0.10 this doesn't apply to you.) So your ROI can actually be higher on Yahoo in some cases. Google does have more traffic, but the key point most ppc marketers forget is, you're not after a high volume of traffic necessarily, you're more seeking converting traffic.