[jandrese]

You don't have to use every address in 10.0.0.0/8 to effectively fill it up. If your corporate policy is to assign a /16 to each floor of a building, and you have a LOT of buildings it's pretty easy to fill up the space even if most of the /16s are sparsely populated. It's much easier to move on to the 11. space when you build that new building that pushes you over than renumbering your entire corporate LAN.

[xxpor]

Right, but that's not relevant for 1:1 NAT (well, at least it doesn't have to be). Since the NAT would happen in software, you're no longer constrained by subnets being physically under routers. 11.2.3.0/24 could contain 10.0.1.0/24 and 10.128.128.0/24 without any issues, assuming they don't use in total more than 256 address.