Important to keep in mind that when Hitwise/Comscore/Nielsen/etc. measure "Microsoft/Bing powered searches" they're including a LOT of searches that never leave MS/Yahoo's sites. If you do a search on MSN Autos or Yahoo! News or in Facebook's web search, for example, those are all powered by Bing and count toward their share.
Contrast with Google, where very few searches on Google's entire network lead to another page on Google (most of that traffic goes out to other sites like those we own/control). Thus, while 30% of web searches may be Bing powered, the Statcounter numbers are the ones I'm much more inclined to look at as a comparison. 8% for Bing and 11% for Yahoo! seem like plausible figures for outbound traffic sent from those engines.