It must burn Microsoft's ass to spend money with Google...
Nah - the products marketing budget usually gets spent by a media planning agency according to the plans of the customer (MS). It's just a fraction of a fraction that goes to Google. Also, I guess that at least some computers at the Plex are running on Windows machines. So it's a give and take, and they don't care.
As a small publisher I also fear if YPN/MSN start with number game which will restrict/allow only those publishers who has certain numbers of impressions per month.
Nah - that's where a lot of money is to be made. The big publishers cannot provide well enough targeted content for all imaginable niches. Also, they want to attract all kinds of smaller advertisers which demand space for their niche-adverts. They would be stupid to let this money go to G.