It might be good for supply (publishers) short term, but it's worse for demand (advertisers) and for ads quality since it removes personalization opportunities. An open-market like RTB where offer meets demand is more efficient (except for privacy-minded users which aren't even part of the transaction). It's also worse for supply long-term as it ties them more to Google and FB, or to a non-scalable contract negotiation process mad-men style with a handful of advertisers.