Do you know from 3 to 4 years yahoo wants to be patched up with Google but Google is not doing that they have given there partnership for NASA and for Microsoft . Mostly everyone thinking about its profits specially Google ...
Yahoo is desperate not to go under. They are gradually slipping under the radar and their executives know this. This is why there are adverts on UK television right now for Yahoo, they are trying to become "hip" - They will never be able to compete with Google. Google gets name dropped in so many movies and tv shows now that its become a household name. And microsoft is constantly coming accross as the bad guy because of their "shut door" attack on the market
i think, for yahoo, its now a case of damage limitation and trying to stay on the gravy train for as long as possible. but i do think that at some point yahoo will either be bought by microsoft in a last ditch attempt to claim back market share. or a hostile takeover by google
Besides crawlers Yahoo must make a TON of Free useful unique services that beat Google's services - then if Google will not compete in that (i doubt really) they will step by step become more popular.