That's a horrible hack and those people are wrong. Separate your concerns; addresses for addressing, firewalls for firewalling. Using private addresses adds extra complications; what if someone's home network uses the same range and they want to connect to your VPN? What if you merge with another company that's using the same range? What if you want to use FTP or SIP or any other protocol that uses the internet the way it was intended to connect to a server in a different office, are your packets going to make it through or not? You'll observe that private addresses have been deliberately left out of IPv6, for good reason.
B) I agree there are inconveniences and complications with using private addresses in ipv4. But it seems to be necessary thrift in the ipv4 world of rapidly expiring address space; using public routable ipv4 addresses for machines which do not communicate with the public internet is perhaps a luxury we can not afford, even in cases where to do otherwise is inconvenient or complicated.
I imagine the only reason you would use a IPv6 private address is if you didn't have allocated global ones. It's just replacing no chance of collision with some chance of collision.
Yes, but they were a late addition, postdating IPv6 by 10 years, and aren't meant to be used lightly.
IPv6 is designed for easy & automatic renumbering so there's less need to hold on to a specific prefix as an "island of stability".
Just gotta make sure OSPF, (E/I)BGP and L2/L3 drop these ranges though.
Of course, some of these machines actually might have valid justification for squatting on a public IPv4 address. Perhaps their firewalls are configured to drop all packets except those from a handful of "trusted" IP addresses, so a random scanner on the Internet gets no response.
But I doubt that such cases account for the majority of "seemingly unused" IPv4 blocks. What's more likely is that some large organization was assigned a massive block of IPs 20-30 years ago and never found much use for them. IBM owns 9/8. Xerox owns 13/8. HP owns 15/8. Apple owns 17/8. Ford owns 19/8. Several pharmaceutical and chemical companies also own an /8 each, as do some universities. Do they really need 16.7 million public IPv4 addresses? Of course not. But I wouldn't be surprised if they started to sell bits and pieces of their blocks once the price per IP goes up enough.