I am currently listening to a TMCnet Web Summit on VoIP Peering in conjunction with Jon Arnold & Associates. This webinar took place last week. Eli Katz is the first presenter and makes a point I have not really focused on. He says that using peering you can share services across networks and subsequently add value to customers and increase revenue for carriers.
His example was that of SMS and how SMS became truly useful when multiple carriers began interoperating with one another.
Other areas he touched on were security, ENUM and SPIT. I have to go on a conference call soon so I may not get a chance to listen the whole web summit now. I do however suggest you listen to it if you have any interest in voice peering.