This is an interesting article about how publishers are using ad exchanges to supplement the traffic they have on their own sites. The issue is a simple one; if you have more demand than traffic, you buy low on the ad exchange and sell high any impressions against users who may have been on your site but went somewhere else. It's a model that I think can work assuming one thing; the publisher is transparent about the supplementation of the impressions. If they are pretending these are their own impressions, than they can get in trouble. This is something that many sites used to do and I hope don't happen anymore.
Keep an eye on the model; it could work very well over the coming weeks.
If you are a planner you should make sure you are asking the questions of whether these impressions are on the site itself or supplemented via an ad exchange.