said by bklass:As I understand it, IXPs only charge rent for rackspace, etc, and no toll for data capacity.
Well, one can only cram so much bandwidth in a given footprint and equipment investment at any given time so, even if there is no "per bit" charge, you still have costs that scale with capacity - you aren't going to need quite the same equipment, facilities access and services for 10Gbps as you would for 200Gbps.
And there are transit providers who do not take kindly to clients (ISPs) stretching the limit of their traffic volume ratio agreements. Transit providers are in the business of selling globally routable capacity by the Gbps, unlike Internet exchanges that are acting more like a large LAN connecting a bunch of relatively local entities together.