With so much trading now done via ATS, there is no longer any need to list on multiple US Exchanges. Pre-computerized communications and pre-ATS, you would see NYSE stocks also listed on every regional exchange (Boston, Philadelphia, Chicago, Cincinnati, and Pacific were the primary ones). That was mostly to facilitate trading for firms that didn't have NYSE seats as trading on the smaller Exchanges was much cheaper. But now a Company just needs one primary listing so much trading is done electronically via ATS.
IEX goes live in about a month. So far, they have not announced any listings at all, but are expected to do so sometime before launch. They are being very secretive about it all, for now.
