Just because an open ad marketplace is “frictionless”, doesn’t mean these same advertisers couldn’t find these same high quality publishers and advertise on their inventory. The value of an auction is that it bids the value of inventory up. If Fred Wilson’s blog advertising was available in an open marketplace, he could still get a premium for his inventory, and only accept advertisers he wanted to work with.
:LATER: Zach Coelius, who’s starting an ad network and technology company, also responds. Note that Triggit, his company, now has an ad relationship with LiveJournal.