While there is no difference from a technology perspective, there's also no benefit to be gained by interconnecting large networks at a public (as opposed to private) interconnects.
Sure there is. Running one larger interconnect has the potential to be much less expensive and easier to run than many smaller ones. Economy of scale...
For whom? Just as a matter of curiosity, lets say 5 NSPs happen to share colo/FM at various locations across the US. What is the incremental cost to them of a back to back HSSI/FDDI connection? Almost certainly smaller than the lower performance and greater adminstrative hassle of paying someone else in the same facility running a NAP to move the packets for you, which also entails greater risk and unreliability. I would guess viability of the NAP model vs. the private interconnect model depends to a great extent on (a) peering policy and (b) for any given peer, what the traffic between networks is. Alex Bligh Xara Networks