As in what is (unfortunately) actually being used. ISP's have taken so long to even begin to look at moving to IPv6 that stopgaps like CGN have to be put in place. Then, of course, why break what is working so IPv6 is put off even further.
CGN has a real cost. ISPs like T-Mobile and Time Warner have found that IPv6+CGN (either DS-Lite or 464) is cheaper than CGN alone because 50% of traffic can bypass the CGN.
That's actually exactly what this is. Currently coal is the cheapest electricity you can get. However, as the price of coal, oil, and natural gas goes up, and as the price of cleaner alternatives (through research, scale, etc.) goes down, we will eventually arrive at using much cleaner energy. The big problem with this is that those two prices aren't moving fast enough to avoid a global climate disaster we will be facing within several decades.
That's why the current incarnation of Capitalism (finance-based, quarterly-results oriented) is broken beyond repair. Kicking the can down the road sound like a hell of an option if you can be relatively sure there will be a different guy doing all the eventual cleanup.
As in what is (unfortunately) actually being used. ISP's have taken so long to even begin to look at moving to IPv6 that stopgaps like CGN have to be put in place. Then, of course, why break what is working so IPv6 is put off even further.