That site does not give any real reasons as why one should not turn off IPv6, and is in fact geared towards Windows environments.
There's no reason why any service should 1) default to, or 2) insist that IPv6 notation be used when there's no other reason for it. And, there's nothing being used that insists that IPv6 be enabled. And, if disabling IPv6 is going to break things, maybe there should be a warning when one does it.
It is comical that the document states:
IPv6 doesn’t need a DHCP server because it doesn’t use NAT.
I didn't know that you had to be using NAT in order to use DHCP. True, especially at home, we use NAT, but that's not the primary reason or use for DHCP.
There’s no need for NAT. Every computer can have an address that allows it to get to the Internet using the same IP that allows it access to internal resources. We no longer have to try to keep those two networks separate through IP addressing.
Except that we don't want every device addressable and reachable.
Regardless, that article is more directed towards people using Windows, and a lot of those points are not valid in regards to the Linux / Unix world.