I'm certain there are many companies that are doing precisely that - but, your introduction sentence was important, "Owned three /16s" - your company had already developed practices and procedures that assumed everyone had a globally routable IP address, and so it was much more likely that when they transitioned to IPv6, that they would likewise use globally routable IPv6 addresses. I bet there is a good chance that they went to the effort of getting their own provider independent IPv6 space - which eliminates a lot (all?) of the hassle associated with using a /48 chunk from their ISPs /32. I got started late in the game (1998), so I've never worked for a company that had a lot of globally routable space (a /18 at most). We always started out inside in RFC 1918, and saved our globally routable space for internet-facing hosts.

The nice things about RFC 4193, is that while the routes aren't floating around on the internet, the addresses you assign are highly, highly unlikely to conflict with someone else's network, and, in the very rare situation that they do during a merger/acquisition - you are no worse off than if you had changed ISPs and had to renumber your internal network anyways. Per RFC 4193 (Section 3.2.3), if you connected with 100 other networks, the probability of collision is 4.54*10^-09 - so the vast majority of the time you won't have a renumbering exercise to worry about.

Note, though - I'm not suggesting that anybody else adopt this strategy of using RFC 4193, and I actually wish more of my colleagues would start using globally routable IPv6 for their internal networks - particularly so I can learn from them to find out what they do when they change ISPs.