for the same reason that having 1918 space & NAT as one layer of your security is frequently useful (though not enough in and of itself), NAT66 does solve some problems and make things harder for attackers.
it does up the complexity of debugging on your part, which is one reason i'm not a huge NAT fan in general. but security is all about risk/benefit/complexity tradeoffs.
be interested to see what you wind up with, assuming you can share.