I'm not so sure that Marcus's claim is entirely invalid. I think what he is trying to propose is that in the long run the competition that Google will be facing by having another ad publisher like Microsoft, will make things better for site owners as it will give more competition in the ad space.

As we know Google is cash filled with money made from their ads program. If Microsoft is willing to give more of that money back to the site owners to gain market share, Google will have to react.

It's not the money though. If microsoft offered far higher rates for publishers, and far lower rates for advertisers than google does, I'd still be reluctant to use microsoft. The main blocker is that they simply cannot write webapps. The interface for dealing with adcenter is amongst the worst webapps of all time. It's a very very slow painful procedure to get anything done at all. Things you can do in adwords in 5 minutes can take hours in msn adcenter.