There were two things that come to my mind. One Yahoo is pretty confident about its Messenger that it knows even if Live Messenger advertisement is put up then also it wont be affecting its usage. Second thought came that might Yahoo could had been forced under legal frame to display whoseover comes to it for advertisement even if it is Microsoft itself.
But point to be noted over here is that why Microsoft wanted to advertise on Yahoo? May be it is losing its confidence or may be it has accepted that in Internet era it stands no where in front of Google and Yahoo. Coincidently Bill Gates has lost his position of being the richest person after many years few months back.
It is correctly said everything that has begun has an end. Is Microsoft going towards its end?