The problem is that you can't be sure that if you use RFC1918 today you won't be bitten by it's non-uniqueness property in the future. When you're asked to diagnose a fault with a device with the IP address 192.168.1.1, and you've got an unknown number of candidate devices using that address, you really start to see the value in having world wide unique, but not necessarily publically visible addressing.
A lot of people who implemented RFC 1918 addressing in the past didn't actually read RFC 1918. They just heard the mantra of address conservation and learned that RFC 1918 defined something called "private" addresses. Then, without reading the RFC, they made assumptions in interpreting the meaning of "private". Now, many of those people or their successors have been bit hard by problems created by using RFC 1918 addresses in networks which are not really private at all, i.e. wholly unconnected from other IP networks. Those people now see the benefits of using truly globally unique registered addresses. The whole address conservation mantra has turned out to be a lot of smoke and mirrors anyway. The dotcom collapse followed by the telecom collapse shows that it was a sham argument based on the ridiculous theory that exponential growth of the network was really sustainable. Now we live in a time where there is no shortage of IP addresses. Even IPv4 addresses are not guaranteed to ever run out as IPv6 begins to be used for some of the drivers of network growth. IPv6 makes NAT obsolete because IPv6 firewalls can provide all the useful features of IPv4 NAT without any of the downsides. --Michael Dillon