It seems that many Microsoft adCenter members are disgruntled with the company policies. The advertisers are being told to increase their bid prices, which of course is not well taken by the clients. Due to this some have even dropped adCenter in favor of AdWords.
However, it seems that the same thing is being done by Google as well. As even Google's AdWords representatives tell the advertisers to raise their minimum bid prices, so that they can have better traffic. And most often, they give in because they do get an increased traffic.
The subscribers are ready to shell out money for Google AdWords for Google indeed has a high traffic. Though for Microsoft adCenter they can't say that they will get as good traffic, so instead they just drop adcenter.
The discussion continues on WebmasterWorld