I too see large page time from Yahoo users on my websites. However, the number of visits from Yahoo is less than that from Google. Anyway I agree with you in case of bounce rate. Yahoo users tends less bounce than those from Google.
Unfortunately Google traffic does convert much better than Bing/Yahoo and other search engines in my experience.
I think its just a coincidence. Don't remember yahoo having cult followers. Plus all the data I have seen paint a pretty similar picture.
Historically I've seen some evidence for higher conversion rates coming from Yahoo compared to Google. Unfortunately though, because the vast majority of users are still using Google you can't really rely upon Yahoo alone for generating a meaningful amount of business.