I don't have any hard data to back this up, but over the years I've seen some specific types of campaigns spend much more on Yahoo (as a proportion of overall search spend) than is the case for 95% of regular PPC advertisers. For example, franchise lead gen comes to mind, as do miraculous-claim beauty products (for lack of a better word).
Yahoo's audience does seem to be quite a bit less technical. While Yahoo might have around 20% of a normal campaign's ad spend, if you run a campaign for something like usenet, or enterpise software you are going to see it taking up a much smaller percentage of the spend.
And as shorebreak has pointed out, in other areas Yahoo is going to make a larger than average percentage of overall spend (but still smaller than google).