It strikes me as funny that Ethernet Exchanges get more press than VoIP Exchanges. You would think that after 8 years of VoIP, most of the VoIP Providers would be in ENUM or some VoIP exchange. Why? One major reason: so that HD Voice and Fax over IP don't break! But I digress.
CENX is one Ethernet Exchange. TELX has one at 56. Covad explains why they use CENX:The ETHER-EXCH is "designed to give partners the ability to answer the growing demand for Ethernet service without additional build-outs." In other words, you get access to the Ethernet network of those connected to the Exchange.
Obviously there is the revenue angle: "Connecting with CENX broadens Covad's wholesale distribution channel and makes it easier for other carriers to purchase and sell our Ethernet services without compromising our service differentiation."
On top of that is the network reach angel (that still means revenue for Covad (or any other carrier connected to the ETHER-EXCH)): "The exchange provides carriers with seamless access to Covad's Ethernet over Copper services, which provide economical last-mile broadband access at speeds ranging from 3 Mbps to 20 Mbps."
There is also the advantage of Integration of diverse networks. A EVPL here, an EoC there, an E-LAN there - all connected.
Megapath/Covad, Level3, XO, New Edge/EarthLink -- get access to their network reach via an Ethernet Exchange (at a major telco hotel near you).