there are benefits outside of address space, but I'm not clear on what they are. Intelligently designed NAT solves the problem of public IP exhaustion for IPv4.
The real problem, it seems, with IPv6 is that it isn't an extension of IPv4, but a replacement for it, which inhibits global adoption. If IPv6 could handle IPv4 addresses, it might have actually taken over at one point.
AFAIK, it's only /really/ used on cellular networks.