Though Yahoo Search is now powered by Bing, they are doing things on their own. They just rolled out "Search Direct" which is their answer to "Google Instant".

Unfortunately, Google Instant on a test works better:

Yahoo Search Direct Vs. Google Instant

Bing has gained ground against Google in recent months mainly through publicity, advertising and shoving it down people's throats.

Google still has 65% of the U.S. search market. That's down about 2%. Whoopee.

In my opinion, the "other" search engines contribute mainly in one way: Google knows they can't rest but have to keep working - hard - at improving.