Re: @boxplayer - "Nobody uses it..."
> it will getting more expensive as time passses by
Running a dual stack network costs more than running a single stack network. If there were a defined end-point for running dual stack, then the cost could be quantified and limited. But there is no defined end-point, so the cost is unbounded.
Dual stack is not a "transition technology" in the true sense, which is to make a change to your own network. In other words: "I want to migrate my network from IPv4 to IPv6. So I will run IPv4 and IPv6 in parallel for (say) a month while I do the changes, and at the end I'll have an IPv6-only network". Dual stack would work very nicely for that.
Unfortunately, ending up with an IPv6-only network is a non-starter. You would be disconnecting yourself from the vast majority of the Internet. There is no clear benefit to starting the transition if you cannot finish the transition; but there is a very clear cost.
Of course, one day, it may be possible to actually transition a network from IPv4 to IPv6. When that time comes, those who waited will find a wider choice of less buggy IPv6 implementations to work with.
Despite what some people have said here, people aren't stupid when it comes to their own money. They *will* happily invest their own money if it will save them money. But that's not something IPv6 offers today. Just repeatedly saying that it will doesn't make it true.