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Large ISPs should probably be interested in access to content, without it their users could find the Internet a very boring place.

Not only that. They are also interested in peer's customer population gaining access to content (and ads) published by their own customers.

In other words there is a lot more money to be made by peering openly and having excellent crossbar-like connectivity through a lot of peering points than by worrying about ways that peering with you enables your competitors to sell more circuits. I have heard that there may be a panel debate on this subject at the upcoming NANOG meeting.