Microsoft Live Search: New Dog, Old Tricks

It's been about a month now since Microsoft launched its Cash Back promotion around its new search engine, Live Search.  When I first saw the promotion, I found it to be an interesting concept.  Setup a standard affiliate program (although MS probably gets better rates than others) and pass back the affiliate commissions back to the consumer.  However, in looking at the rates on the Cash Back offers vs. the affiliate rates posted on their retailers sites, clearly Microsoft was giving more than standard affiliate rates.

In actuality they're giving consumers a portion of their ad revenue. They are applying a new model to search by collecting payment from advertisers after SALES instead of CLICKS.   This is clearly a strategy to differentiate from Google and lure advertisers to the Microsoft engine.  But my question is, what is the sustainable advantage for this model?  If this turned out to have traction in the market, Google could simply create a similar program to eliminate any incentives advertisers have in going to Microsoft.  And all this would lead to eroding profits in the online ad world as all the search engines one up each other with cash incentives.