If you think about radix tries and other data structures (AVL) it would be useful to be able to code good x>y tests against the binary state of IPv6. If you have to segment into two u_64 then shift-and-mask stuff becomes harder. So, purely for IPv6 (and I work with internet addresses a lot so my bias is showing) I think this makes sense.

But I totally get it may not be in your use case or first pass. Tiny home router builds? they'd want this. Anything doing IPv6 packet work testing or finding IPs in a structure will want this unless people code to the /64 model.

Yes, I agree.