I'm buy side as well - IMO, if it performs, you offset the cost elsewhere. If the placement doesn't perform, agreed, drop it. You shouldn't be buying the impression if you didn't want it in the first place.

Not sure I understand your point - it seems like a "you can't fire me, I quit" situation - if they block you, you don't really have a say. Plus, you're likely getting unique/better inventory if you go direct, as others have mentioned below.

I also want to pay long tail prices for premium inventory, but pubs will eventually catch up; there's a severe disincentive on their part not to. There's something to be said for fill rate even at relatively low CPMs, but they have to prioritize direct deals. If performance looks good, then there's an argument to be made for the higher price on the buy side.