Many analysts are starting to wonder about this and for some rather obvious reasons.
Neither Yahoo or Microsoft has been able to catch Google in search. Even with their share of the market combined now they still trail Google. Many are wondering if they will do much better as a team.
The same is true with advertising. Both of them have been trying to come up with something like AdSense and neither has it mastered yet.
A benefit is that Microsoft and Yahoo would no longer need to compete against each other and could devote their full attention to Google, but previously each of them was still more interested in doing battle with Google than with each other.
Here is a link to a very interesting and detailed article that examines all of this and many of the good as well as bad points of this potential deal
Would a Microsoft-Yahoo deal out Google Google?